The rules have changed so many times over the years, if she won't sleep on her back, then I wouldn't push the issue. Every baby is different- the reason for concern is that a baby might suffocate in their bedding while on their belly. My mother told me that when I was a baby, doctor's suggestion was to lay a baby on their belly for fear that they would aspirate and drown on spit-up while on their back. Doctor's are really overpaid, educated guessers. Just be sure that there is no bedding to suffocate her and lay her how she is comfortable. My son is 10 months now and the fear of SIDS is practically gone- I am sure you will probably have many restless nights up just watching her breathe, I've been there! My son wanted to lay on his stomach from time to time and the fear of the doctor's warnings played in the back of my mind, but he was more comfortable that way. It's really your call if you want to keep trying, but it seems that she is more comfortable that way. Something about the body heat on their belly actually
can reduce the symptoms of colic (I don't know if you've had the pleasures of dealing with that, I did!).
My suggestion is to make her comfortable, take extra care that there is nothing for her to suffocate in, and let her sleep!
If you are really concerned over fear of SIDS- do some resaerch on the net- there is a special bed cover that will trigger an alarm if there is suddenly a lack of movement in the crib. (like stopped breathing) I can't promise it's cheap or even widely available, but it's out there. I know as much as anyone that peace of mind is priceless, so take a look!