It sound like you are just hitting somthing other than the wood stud. It could be hitting a nail, pipe, conduit, etc.

Try moving over and up or down about an inch and see if you can get the srews in. If you can’t get them in you need to try to figure out what it is that you are hitting that is preventing the screws from going in.

behind the plaster is a sealing foam, now probably dried, but still thick and stuff. perhaps this is the problem. also, (if you haven’t done this) it is procedure to before putting any screws or nails through your walls, you should check and see if there is a piece of wood behind the plaster. a lock will not keep if the only thing keeping it in place are screws secured in nothing. well, hope I helped!
good luck!

If by plaster you mean drywall, then you should be using drywall anchors. You can then fire a 2-3″ ***** through the wall into the stud, which is probably what you are hitting. If you are not sure if you are hitting a stud, then don’t try it. You may hit a conduit or water line.