You're on the right track ...
You seem to be missing setting;
MPNowPlayingInfoCenter nowPlayingInfo
Without it, you will get the results described, IE after pressing pause, the lock screen no longer shows the pause, or indeed that it is playing a song. Here's a guide on how to set it (i've taken this from working code I did some time back, but I'm sure you can figure out what's what).
MPMediaItemArtwork *artwork = [[MPMediaItemArtwork alloc]initWithImage:albumImage];
[MPNowPlayingInfoCenter defaultCenter].nowPlayingInfo = [NSDictionary dictionaryWithObjectsAndKeys:aSong.songTitle, MPMediaItemPropertyTitle,
aSong.artistName, MPMediaItemPropertyArtist, artwork, MPMediaItemPropertyArtwork, 1.0f, MPNowPlayingInfoPropertyPlaybackRate, nil];