Apple announced that it has been underestimating the number of readers using the Apple News app to read news from more than 100 publishing partners. The company is working on fixing the problem, but for now, neither the publishing partners nor Apple know how popular the app has become. Publishers need an accurate number to manage their advertising strategies. Executives at the publishing partners have been underwhelmed by the reported traffic from the Apple News app so far.

The Apple News app launched last September with the release of iOS 9. Apple says that 40 million people have used the app, but the publishers aren’t impressed. “The traffic has been modest relative to the enormous install base of iOS devices,” said Business Insider president Julie Hansen.

Business Insider, The New York Times, The Washington Post, Vice Media, Time, The Wall Street Journal, and The Atlantic are just a few of the major publishing partners. These companies are eager to take advantage of the large audience for news on mobile.

They can also make some ad revenue. Publishers can keep 100 percent of the revenue when they sell their own ads for the app, or they can let Apple sell the ads and 70 percent of the revenue will go back to the publisher.

Publishers have actually called on Apple to sell ads frequently, so the company is in the process of improving its own ad-selling network. The Wall Street Journal reports, “Apple has accelerated the development of its iAd network and expects to launch a self-service ad-buying platform in the next two months to help increase ad spending.”

The company is also integrating comScore measurements at the request of publishers.