M.R.
My daughter was a LATE roller. Actually should could go from stomach to back early (2 months) and then was late on back to stomache (7months). I had the Parents As Teachers organization that was visiting and they were showing me all different ways to help her learn that movement and she refused to, she stuck out her arms and would not let us move her. But as soon as she was ready she rolled over and started to crawl all within a day. Kids work at their own pace. My daughter was an early walker at 9 months so it all goes to show that they will do what they want when they are ready.