Weak openings this year for new installments in several older movie series highlight a challenge facing Hollywood studios: Even familiar titles often have trouble finding audiences—with the notable exception of superhero films.

Sony Pictures Entertainment’s “Charlie’s Angels” reboot—based on the television show about three crime-fighting women that first aired in 1976—bombed this past weekend, opening to a pitiful $8.4 million in the U.S. and Canada. This comes just a few weeks after Paramount Pictures’ “Terminator: Dark Fate,” with Arnold Schwarzenegger reprising his role from the original 1984 blockbuster, grossed a paltry $29 million during its first weekend.

It is a growing trend, according to entertainment analyst David A. Gross, who says new franchise installments released this year have grossed on average 33% less than their predecessors for their opening weekends. The decline from one installment to the next averaged 16% between 2012 and 2018, he said.

“There is going to be some falloff for sequels, remakes and spinoffs from episode to episode,” said Mr. Gross, who runs the movie consulting firm Franchise Entertainment Research. “But they are opening significantly weaker in 2019.”