Our son has quite bad plagiocephaly (official name for a flattening of one side brachycephaly is where you have a flattened head across the entire back of the head).
If you have a flattening on one side as our son as you must try and turn their head to the other side as much as possible when they're sleeping. Wait until they are in a deep sleep before trying to. It won't flatten out that side too because you'll find that he/she will still go back to their favoured side. Sometimes feels like fighting a losing battle to me.
How old is you LO?
We first noticed it in Isaac when he was about 6 weeks old but didn't know much about the condition at that time. Our GP said it would correct itself which is the standard response from the NHS. Rubbish, it's got much worse now. They didn't mention repositioning to us at all. I only wish they had of done.
A sleepcurve mattress will help also as they're designed to help prevent this problem as it takes the pressure off their heads. You can get them for moses baskets or cots. We're struggling to get one at the mo though.
During the day time keep them off their backs as much as possible. That can be difficult when they're very young as they often just want to sleep, especially prem babies (as our son was prem).
Put them in a bumbo seat if they're old enough, tummy time or in a sling, baby carrier. Anything that takes the pressure off their heads.
Keep a close eye on it, if your baby is young enough and it's not too bad you may well stop it from turning into a bigger problem.
Search for plagio on google and you'll find a couple of excellent forums that can give you good advice on this.
Isaacs head is badly misshapen now, I really hope you can stop your LOs flat head from getting worse. We're having to seek private treatment now to correct it.
it dosnt happen to all babies 
