13 month old forgotten how to roll over?

[GiveMeVegemite]

My DS can stand, cruise, sit, crawl etc but when I put him to bed at night he can't roll himself over from his tummy to his back! I see him in the video monitor swinging from side to side and then sitting up and then trying to get onto his back, but he gets his arms caught down by his bum and can't roll!

He was never a big roller, but he could roll when he wanted to (mainly at night), but suddenly he can't. He is completely normal with his development, has no physical problems etc so I just don't understand!

Any advice?

My 15-month old doesn't roll very often now - she has taken to sticking her arms up for me to get her back up when she's lying on her back! She was never a big roller and only rolled once she'd started to crawl, so I think she just doesn't like/isn't keen on it?
She can roll, and has begrudgingly done it a couple of times. I'm not too worried about it, as she is walking, running, standing up etc. Maybe it's just one of those things?!

Thanks for the response. Hopefully you're right. He was never a big roller to begin with! :)