Flat head, uneven form of head or whatever you call it, there is a big tendency (well, according to my fore-parents, grandparents, close family circles) and I can feel it that sleeping is one way of causing such. I know a baby has its own pace but parents has also have to take good care what is a responsibility on their hand, especially the mother or a caregiver which is being close to a child. Being close as she is watching her all the time, she is with the baby all the time. This means that she must also has to watch closely how the baby turn her head while sleeping, or the way she puts her head. When you help her balancing her head (balancing in a way of every 30 minutes or an hour or so, when she puts her head in the right direction and does not put in the other direction just as she puts in the former, then you have to help her getting used of putting her head on the other direction too, or balancing. In this way, her head is balance when she's already big. You know a child's head is so soft so any form she's used with it, the way she does it will also go with the flow since it is still soft and manageable.

So I suggest you to take closely of your child's head when she's asleep and do what you can do to let her head be even! Put or Lye it in the opposite directions one at a time!