forgot how to roll over?

Fa

So my baby is gonna be 5 months old in a week. He's been rolling from belly to back for about 2 months now, and about 2 weeks ago, figured out back to belly. so when he's playing in the floor, he rolls to his belly and gets frustrated after a few minutes. It's like he forgot how to roll back on to his back. I've let him lay and try to figure it out on his own, but he gets so frustrated he's doing some real crying. Anyone else experience this?