Rig Deforming my model?

The body moves just fine but when I try to move the head this happens:
I have no idea why this part wont move along with the rest. I am able to move it as a whole by itself, but as a rig it just does this. Is something wrong with my mode or my rig?

Take a look at the rig in weight paint mode, you will probably discover there are some verts left out of a bone’s influence, so when that bone moves, the verts are left behind. See tutorials or documentation for weight painting to fix the problem if you find that is it.

Ah okay, thanks. I looked in weight paint mode and it was colored different from the rest so I messed around with it a little. Thank you!