I agree with julia's advice on where to place them.

the only things I found that worked for the 2 weeks of hell that we went through with the identical issue was breastfeeding her almost to sleep them putting her down straight away (we reversed our routine from feed/play/sleep to feed/sleep/play for just these 2 weeks) OR (and this worked better) was just to keep her up for quite a long time after a feed until she was just about falling asleep in our arms, then put her down.

after 2 weeks she was still rolling and getting stuck but had learned to like being on her tummy, once I realised this it was easy as I'd put her down on her tummy and then pat her back until she was relaxed - she stopped rolling then - until she woke up!

now we just have her putting her bum in teh air and pushing along on her head! until she hits the end of her cot!

sids from sleeping on their tummy is not really a risk once they can roll and hold their heads up- but wrapping a mobile baby is a bit risky and bumpers are a risk as it's not just a suffocation issue but also the fact that the bumper restricts air flow into the cot and this is associated with an increased risk of sids.

don't forget lots of tummy time when she's awake adn toys just out of reach to give her incentive to learn how to roll back. they get much happier when they can roll back too.