After you've tapped a track in the main list of tracks to start it playing, tap the skinny "playing" bit at the very bottom. It should pop up the full screen pl

After you've tapped a track in the main list of tracks to start it playing, tap the skinny "playing" bit at the very bottom. It should pop up the full screen player. Is the shuffle icon selected (green)? I had the same issue a while back and I was infuriated at myself for not figuring out the solution.