Adjustment layer effect won't motion track null object layer

Hi all,

I have an odd problem. I Am trying to do something seemingly quite simple - make a person's teeth grow into vampire fangs as he opens his mouth and moves towards the camera.

My null object layer is motion tracking perfectly and I am using liquify in an adjustment layer to distort the teeth. Then I am using the null object as the parent of the adjustment layer - but the distortion remains resolutely in the same place! 

To double check that I was doing everthing right I added a text later and used the null object as its parent, and the text moved perfectly. 

I'm very puzzled. 

I'm probably not doing something really obvious but I can't for the life of me work out what it is!!

Any help gratefully appreciated...

Thanks
Sean