Well Joey,
there are several reasons why a student *do not followthrough*- but how far is not the real problem.
He has to setup correct and finish the stroke- How to explain this can be different on several students.
For example often it is enough to video them while executing- and after that again by just letting them shooting the cueball with a higher speed into a pocket- and from my expirience in the most cases they see immediatley themselve the difference. If they re just shooting the ball (without any objectball) into a pocket with a higher speed (4-5 is enough) they all finish nicley
There also can be several types of shot, where you would adjust a bit different. This here is just one example from my side and expirience i detected mostly with beginners or less advanced players.
lg
Ingo