Try to get her attention by rattling some rattle toys at her side. Encourage her to reach out for the toy by her side. That was how I trained my boy then. My boy did his first flip at 4 months+ and that was the one and only flip he did. He only started to flip actively when he reaches 6 months.
Encourage her with lots of smiles and praises. My boy hates to be on his tummy so i think he figures that it was no fun to flip, cos he would end up on his tummy which he hates! dont't worry, babies will flip when they are ready :-)