Song available on web app, but does not appear in ios app

[beashi]

Song in question is Trey Songz' Flatline. I've tried searching for it on the mobile app and I could not find it. Decided to search for it on PC and lo and behold, it's there. I add the song to a playlist using the desktop app and I go check my playlist on mobile and the song isn't there. It's so weird but is there a particular reason why this would happen?