My bb neck strong since he bout 2wks old... Always mvg n liftg his own head... But my bb nvr mv up like urs.. He slides dwn... Hehe..just have to keep a lookout when bb are on their tummy. its v normal to place them that way. there's alot of bb grew up that way.
m 2 nieces also litat. i myself also...
my girl also moves upwards like a worm moving when she's on her tummy. ha.. just make sure they dont fall off the bed..