Sometimes the shell holder or lock stud has a burr somewhere that prevents the shell holder from pulling tight. Try turning the shell holder on tight without a case in it. The stud has to pass through the hole in the shell holder to contact the case base. If the lock stud passes through with one shell holder and not another, then the shell holder is the problem.
Return the shell holder with a note of your issue to the factory and we will replace it.
LEE Precision