You could try going to a paediatric physio. My friends baby had the same problem. She had one side of the neck with tighter muscles (due to her position in the uterus) that's why she favoured sleeping on one side and not the other developing a flat head. My friend was referred to the physio by her child health nurse and it cost nothing. She was given a set of exercises which fixed the problem very quickly.

Both her GP and Paed just dismissed the problem but the physio actually fixed it.