For me, playback of new songs often just doesn't happen on iOS. The app doesn't start playing the song at all. I need to like the song which I think makes the song available offline. This usually helps.
My main gripe with the iOS app is how they handle offline playback and support. There are two really distinct offline/online modes in the app and you will often have to explicitly switch your phone to airplane mode in order to kick it into it's offline mode and get access to your downloaded music. If you don't do this and happen to go through some patchy network area (like for instance the stairwell in my building) it will continuously try to load album artwork etc. and block rending of the rest of the UI so you can't even hit play on audio you've already cached/downloaded for offline use.
