Problem with Maya animations

When i created my game character in Maya, with animations working fine within Maya, soon as i begin to test the characters animation, the neck seems to disappear, so the head is hiding in the shoulders. anyone know why? any advice?

Hi, im creating a game for a college project, and when i created my character in Maya, the animation worked fine. everything worked. i then imported it into Unity, made sure i had all of the animation on the avatar, and as soon as i press play the characters head sinks into the collar bone, the neck seems to disappear. only the animation works fine. is it something to do with the model itself or is it an issue with Unity? Help!