Hmm ok, the same code is used for both desktop and mobile.
So if it works on Desktop, then it should (will) work on mobile also unless you have some specific caching active.
In a rare case your visitors may be super “unlucky” that the same advert is chosen over and over. 3 adverts in the group means that each advert has a 33% chance to get picked for that page load. That’s a fairly high chance. So it’s also entirely possible that things are just working normally .