We have put up a new drywall ceiling up over a plaster one... the pot light holes are cut and wiring in place. The problem is that the pot lights will not sit properly because the clips don't clip into place. I think it's because of the thickness of the drywall over the old plaster ceiling. Any suggestions!!

Would be great to know the make and model number of the recess light you have, and the thickness of the ceiling material, and any other details that maybe helpful.

I don't think they will work with that thick of a ceiling. The exposed ceiling needs to remain tight for the can, you may have to remove drywall wher the clips make contact so the thickness from the surface to where the clips make contact is abou 1/2- 3/4".

Forget about the clips. Drive drywall screws through the can into the surrounding material. Four to six 2" screws should be plenty. If you can catch a joist even better.