My DD only rolled over 3x from tummy to back and that was a few weeks ago. She doesn't seem to want to do it again. She's in physical therapy, and the therapist gave us some tips to encourage her to learn to roll. One is to gently pick up one arm while on tummy, and the same leg and roll her over. Give a big hand of applause, and roll her back onto her tummy. We do a diaper change on the floor first, then I roll her onto her tummy. When she's finished with tummy time, I roll her back onto her back. If nothing else, it gets her used to the sensation, so she won't be too startled or freaked out when it happens again.