Hey Renee - Dani (Oshani) little Oscar had this problem too as he favoured one side alot and she took him to the chiro and that has helped - there is a thread around somewhere that I will try to find for you. One of the girls at mothers group used an osteopath for her daughter.
Sometimes because of the way they lie in the womb - thier little necks get a bit tight on one side so a bit of gentle treatment can help loosen it up and make it more comfortable for them to sleep on the other side.