I've made this tiger head rig, and started adding teeth. When I started to rig those teeth (as separate objects), I noticed they did not move with the head bone in a similar fashion. This was the case, however, until I noticed that the jaw/mouth itself was deforming when the head bone rotated, making the teeth not stay within the head. It does this for any direction.
^Every bone appears to be linked.
^Viewed straight-on appears normal.
^Rotating head makes it slack-jawed.
^Weight painting seems fine to me (but I'm very new). I also painted the teeth, but they don't deform strangely in the same way.