Employees at Yelp have seen a troubling trend recently: Facebook sometimes appears to be blocking their app. Even if customers have downloaded Yelp, links posted to its Facebook page direct them back into the social network’s own browser. Instead of sending viewers into Yelp’s restaurant review app, they stay stuck in Facebook.

Yelp says that traffic from Facebook isn’t material to its business, and isn’t sure of the cause. But the broader issue is clear: The potential for so-called deep links to tear down walls between apps has diminished over the past year. Deep links—intended to make links to content in mobile apps as shareable as web pages—that were posted to Facebook haven’t redirected people to outside apps on iPhones for months, said Alex Austin, CEO of Branch Metrics, a startup that creates deep links for companies like Yelp, Target and Jet. Those links used to drive users to outside apps about 80% of the time last fall, but now only work when brands buy ads.