One of my boys did exactly the same, he could but blatently refused to roll back over, it also drove me insane esp when he did it at night. You can get pillows that stop them from rolling over but they have been linked to cot death so i decided against it. There is light at the end of the tunnel when my lo was about 7 months he started rolling back over. its funny, but until i read your post i had forgotten all about it, even tho at the time it consumed me lol good luck
xxDBxx