Dare I say try letting her fall asleep on her stomach? And then going in to roll her over back to her back once she is asleep?
Jenna sleeps on her tummy/side all the time now, and yes she is 19m old, but even then, we were starting to let her sleep on her tummy fairly early, and just going in and rolling her back if need be.
Just a suggestion. Goo dluck!