Almost Rolling Over

Last week Jubilee rolled over on the bed from her stomach to her back. She did it three times, but when I laid her on the ground to do it, she just cried with her face down on the blanket. I figured the bed gave her an advantage--you know, those pillow tops are softer than a blanket on the floor. Anyway, I caught her doing this today. She's almost rolling over, but not quite. She keeps getting her right arm stuck under her, then doesn't know what to do.