Mine sleeps on the tummy during daytime since 1 month old.. He sleeps deeper rather than on his back.. But at nite, have to let him lie on his back to ZzZ.. Mine currently trying his utmost strength to flip but can't... Kinda funny..
His neck not strong enuff to lift 45deg up when on his tummy? Maybe he's not used to it.. Can try for few more times, he will get used to it..