Your problem might be a simple one. Hopefully so:

Run a business card around the edges of the screen between the screen and the plastic housing to see if there is any trapped dirt or other debri. If there is, it will hold down that point of the screen and prevent any further input. Be sure to remove anything you find.

Also, sometimes the screen protectors themselves will shift just enough to cause the same issue. Be sure it is not wedged tightly between the screen and case.

I hope this is it and helps!