Mobile app developers on the Windows Phone platform were reporting a sharp drop-off in ad revenue from Google's AdMob ad network, which has led some to speculate that Google was aggressively trying to kill off the rival operating system by making it harder for Windows Phone developers to monetize their apps, Windows blog WMPU first reported.

Google has since said, via tweet, that "we fixed the Windows Phone issue in AdMob and fill rates should be back to normal. Thanks for bearing with us."

Google's AdMob is one of the largest mobile advertising platforms. It allows developers across a number of platforms including Android, Windows Phone and iOS to earn money from their apps by integrating an SDK (software development kit) which serves ads.

Earlier this month, developers noticed a suspicious drop in ad revenue from their Windows Phone apps, which, incidentally came around the time of Microsoft's annual "BUILD" developers conference. One of the big announcements at BUILD was that Microsoft was going to make it easier for developers to port their iOS and Android apps over to Windows 10.

Writing on a Google Group for mobile ads SDK developers, one developer claimed the fill rate on their Windows Phone app/s (the amount of ad space actually sold) dropped from 99% to 7% in one day, starting May 7. Another said the revenue generated by AdMob ads had decreased by 80%, and other forum members said they were experiencing similar issues and had yet to receive technical support from Google.

Some developers also tweeted about their Windows Phone/Ad Mob experiences:

On Monday, a Google spokesperson sent Business Insider the following statement:

"We are aware of reports of fill rate issues for Windows Phone developers in AdMob. Rest assured, we are actively investigating and working to fix this, and hope to have more details to share soon."

One member of the Google Group suggested the issue may be down to a bug, while others were more conspiratorial as to the reason behind the drop in ad revenue.

"Luis," wrote: "Sure, the google [sic] is trying to kill the WP platform (and this huge drop was after the Build 2015, coincidence?), that's obvious. Guys, forget the AdMob, use the [Microsoft] pubCenter, is really better than AdMob."

A commenter — Scott Humble — on the WMPU blog had another theory: "I am not convinced that Google is competent and calculated enough to do something like this. An unannounced Google API or service change seems more likely. It is possible that this is anticompetitive but we would need comparative data from competing mobile ad services as well."

In the meantime, other mobile ad networks such as Microsoft's own PubCenter, or independent networks like Smaato could provide alternative sources of revenue for Windows Phone developers.