I've noticed that sometimes Spotify stops playing. I play music from my iPad, via AirPlay to my Apple TV (gen 1). To be more precise, the song keeps playing when the iPad screen goes black (I've got a 2 minute lock/sleep screen setting). But when the next song is supposed to start, nothing happens. From my woken up but still locked iPad, I see artwork and controls but when pushing the play icon, it does not turn into a pause icon and the song never starts. Manually unlocking the iPad makes it possible to keep playing. Then the issue reappears next time the iPad goes to sleep.
I've noticed that it does not help closing all other apps. A restart of the iPad is necessary and this will fix the problem. I have many other apps on my iPad and I don't know at which point this issue starts to appear.
I'm on iOS 8.4 with the most up to date Spotify (188.8.131.529).