Barb....I had this issue with Olivia......she would happily sit, but never move, and if she was on her tummy, was very happy, but couldn't /wouldn't roll over.....

I know the jury is out on Jolly Jumpers, but I think it did help her feel a little stronger.

But more effective than that was LOADS of playing on a soft sureface....we ended up taking the double mattress off our spare bed and laying it on the floor and playing a LOT of rough and tumble games with her and this is what helped her start rolling, and simply feeling more confident in the gross motor area........

(That said, she still didn't crawl....only ever bum shuffled......and didn't walk till 19 months....)