Hi!
From the looks of it, youd Jaw Skin is not fully influenced by the Joint.
Do you have "Use Color Ramp" turned on in the "Gradient" section of you paint skinweights tool? This option makes it easier to see smaller weight differences. In black an white mode a part of the mesh may appear like it is 100% white when it is actually just a really bright 95% grey.
I would also select the vertices of your jaw, set Opacity and Value to 1.0 and the use the "Flood" option to fully skin that part of your mesh to the jaw joint and see if that makes a difference.
If that isn't the case, the only this I can think of that would cause this behaviour, is that you have another deformer (lattice, clusster, non Linear, deltamush.) on your face mesh that is partially influencing the bottom of your mouth. In that case you would either have to replicate this deformer for the teeth as well, or just give some of the influence of your teeth to the head joint.
I hope it helps!
