The lock cylinder is either worn out or broken.
Answer your second question...all the lock cylinders are
the same...so yes you could swap it over.What good would that do though? Your still left with one bad lock...I would remove the bad one first...maybe something has come off like the inner clip? I don't understand what you mean by slipping? Do you mean the cylinder itself is moving in and out? There is a spring loaded clip inside the door that holds the lock in place and maybe it has fallen off and would be inside the door on the bottom, you could check that when you remove the panel.