We had the same. Once LO got to 7 months though she just rolled tummy to back one day and that was that. I tried teaching her but it just took her a while to master it. It will come. She still rolls onto tummy and cries at night sometimes though. Every other morning I wake up to find her on her tummy with her head to the side. She seems to enjoy sleeping in this way and puts herself in that position.
Look at highlights reel on milestones and motherhood on instagram - loads of useful stuff on how to teach your baby to roll both ways to work through that sleep practice!