I used the German store to purchase a song. I've tried to switch back to the US store and my account shows that I have, but when I try to purchase anything I'm told that I have to switch back to the US store. If my account shows that I'm with the US store, where do I go from there. It's making me crazy!
I had this problem a few months ago, just go to Settings > View Apple ID > Country and go through it all again, so select US, then keep going and verify your payment info and it should stop it - I haven't had any problems since I did it with mine.