Sleeping on back

My LO (12 days old) is struggling to sleep flat on his back. he leans from one side to the next. Should I be worried about this? We have the Graco pack n play Day2dream sleeper. He has never slept in the pack n play portion, although that was our intentions from the beginning. Instead he has been sleeping in the bassinet portion. My main concern is that it is slightly (and I do mean slightly) elevated. It does not go completely flat. My goal is to get him to sleep in his pack n play since it's completely flat. Any tips would be appreciated.