In recent weeks, there's been a lot of scrutiny of recent projects by established pop superstars.

Justin Timberlake's kick-off single from Man of the Woods, "Filthy," stalled at No. 16 on Billboard's top 40-based Pop Songs airplay chart and the follow-up, "Say Something" (featuring Chris Stapleton) is still waiting to reach the top 10. Taylor Swift's "Look What You Made Me Do" went to No. 1, but didn't linger there, and subsequent singles have stopped short of being consensus hits. Somewhat similarly, P!nk's "What About Us" hit No. 10 after a three-month climb.

Given the nature of pop radio, career trajectories are under constant scrutiny. Swift's Reputation is still a 2-million-selling album, according to Nielsen Music, and one of the few that is. But producing fewer hits than her previous 1989 is the subject of this week's Billboard Chart Beat Podcast with Gary Trust and Trevor Anderson. And Timberlake's stumble prompted an e-mail from a reader asking if there was a turnover in pop's core artists. There's a lot to consider.

There Are Few "Automatics," If Any: Top 40 radio is tighter than at any time in recent memory. In part, that's because the mother/daughter coalition that powered the format for a decade is fracturing. There's not much consensus on anything. Has mainstream pop been upstaged by hip-hop and R&B? Most of those hits are struggling for top 40 airplay commensurate with their stories elsewhere. At this moment, there are few "automatics" -- acts guaranteed to go top five each time out. You can write a story about radio not bringing Timberlake's recent hits home, but you can also write it about the resistance to the last few Chainsmokers singles.

Longtime Hitmakers Are in a Trick Bag: Why the era of fun, uptempo pop beloved by both mothers and daughters is waning is a column unto itself, but there are clearly challenges for the artists who defined the "turbo-pop" era of 2005-12 (or thereabouts). Were listeners unwilling to accept social commentary from Katy Perry or P!nk? Would they have been any kinder to more "Last Friday Night (T.G.I.F.)"-type frivolity, when that's in short supply anywhere at top 40?

Timberlake's retro-disco experiments ("Suit & Tie," "Take Back the Night") yielded only one undeniable smash hit in "Can't Stop the Feeling!" Those songs all lived in the shadow of "SexyBack" and how that song widened the definition of pop music more than a decade ago. But the reaction to "Filthy" shows that not everybody was ready for Timberlake to bring edgy back, either.

Swift, in particular, is facing the difficulty of pleasing everybody. After Reputation, it was possible to hear from people both that she had too thoroughly repudiated "the old Taylor" and that the new album wasn't different enough from 1989 and previous projects. It's worth considering that the moody, downtempo EDM sound that Swift helped bring into the pop mainstream is still paying off for other artists; Halsey's "Bad at Love" is both a sonic and conceptual successor.

Multiple Tracks Send Mixed Signals: For years, it was a common strategy to release a second radio single just as an album reached street date. It sent the word that this was an album with depth, and one worth purchasing. Swift took that further by releasing multiple tracks in advance of her albums. In doing so, she was protected by radio's general lack of enterprise on anything not promoted to it. Radio in New Zealand played 1989's "Out of the Woods" as a pre-release track; American radio waited until it became a single 15 months later and then showed middling interest.

The difference this time was radio's lack of long-term love for "Look What You Made Me Do." That allowed quick follow-up "…Ready for It?" to live a chart life of its own, and emboldened some PDs to back off the first single. "Gorgeous" muddled things further. Then as the focus switched to "End Game" (featuring Ed Sheeran and Future), word spread among programmers that "…Ready for It?" was starting to show research stories. Even "Look What You Made Me Do" now has some research stories, possibly because listeners didn't get to hear it very long in the first place.

It was similar for Timberlake. As radio grappled with "Filthy," suddenly there was "Supplies" at rhythmic radio. Then "Say Something," which gave pop programmers the further confidence to cut bait on "Filthy." Katy Perry had gone through three Pop Songs entries by the time Witness was released. Last time around, the pre-release tracks had included "Dark Horse," which later took hold as the real hit from PRISM (other than "Roar"; both topped the Billboard Hot 100 and Pop Songs). This time, it created a sense that an act was scrambling.

In the early '00s, it occasionally happened that an artist, after a disappointing kickoff single, would put a project on hold for six months. (It happened more than once that "internet leaks" were blamed.) That gave Usher a chance to put the largely unsuccessful "Pop Ya Collar" behind him and start fresh with "U Remind Me." It's also worth noting that despite the middling reaction to Justin Bieber's fall 2017 team-up with BloodPop "Friends," his hitmaker status isn't part of this discussion, in part because there were no other singles on its heels.

Are First-Day Premieres Hurting? They were radio's way of trying to hold on to the music discovery mantle as well as publicize its "artist initiatives" in conjunction with record labels. But does hearing a new single hourly on the first day of release diminish, rather than amp up its excitement? Does it allow listeners to decide they don't like a song before they've had a chance to live with it? It might sound odd to think of any song as high-concept as "Look What You Made Me Do" as a song that needed a chance to grow on listeners. But it did, in fact, develop at least a few research stories after running its course.

By comparison, consider Maroon 5's "Wait." It's been over six years since "Moves Like Jagger" (featuring Christina Aguilera) propelled the band back to pop image artist status. A new Maroon 5 song could be either a consensus hit like "Don't Wanna Know" or a near-miss like "Cold" (top 10 on Pop Songs, but not a true power). "Wait" is up to No. 12 so far on Pop Songs; however far it goes at this point, it seems like a perfect example of an unassuming song that needed to grow on listeners; it did not get a first-day premiere at radio, and it might be better off for it.

Radio Still Wants to Play These Acts: Of the acts under discussion, there's seemingly nobody that radio doesn't want to play. That's why they get first-day premieres. Top 40 resisted Lady Gaga's sonic stretches with "Perfect Illusion" and "Million Reasons." But major-market top 40 jumped eagerly at "The Cure," more clearly an attempt to give radio the type of song it favors now. And even though that didn't ultimately become a consensus hit either, some stations like WHTZ (Z100) New York held on to "The Cure" long enough for it to become a hit well after its chart peak.

For many artists, the moment of truth is the first single from the next project. The right song can regain momentum from a middling project quickly, as demonstrated by recent rebounds by Imagine Dragons and now, arguably, Meghan Trainor. Top 40 can certainly reach the point where it is unwilling to fairly consider even the best, freshest-sounding single from certain established acts. It seems unlikely that any of the acts in question is anywhere near that point yet. But the answer is different depending exactly where an act is on a chart trajectory -- are they following up one disappointing project? Or several? And a veteran with a mixed chart record might get consideration on the first single from a project, but not follow-ups.

Swift, in particular, has shown a talent for reinventing her sound. The moment when "Mine" sounded like a less exciting iteration of "Love Story" and "Today Was a Fairytale" bordered on self-parody was long forgotten after "We Are Never Ever Getting Back Together." What she should do next is more of a question mark than her ability to do it.

(Almost) Nobody Retires Undefeated: Assuming they live a long and happy life, there is almost no hit-making artist that will be an automatic add at top 40 radio (or any other format) through the end of their career. It's an unfair burden on any contemporary act to expect them to do what Madonna, Elton John, Prince, Michael Jackson, the Rolling Stones and Elvis Presley did not. A few acts -- the Beatles, the Eagles -- have broken up at a time when they were still having hits each time out. In those cases, we got to see solo members -- Paul McCartney or Don Henley -- have a typical radio arc. That said, to speculate now on how long an act currently at the top of the charts has isn't fair either -- each new song is another chance to extend the hit streak.