Flat head

My daughter is 6 months old almost 7 months. Her head is still flat and we do tummy time and she never really lays on her back, even when she’s asleep she is on her side. I don’t know what to do at this point and I am worried. Is it too late for her to get a helmet? Is there anything I can do to help round it out? It looks a lot better than it did before, but it just stopped rounding out completely and we don’t know what else to do.