Zanussi Door interlock

Was working on an old zanussi machine this afternoon.Stone dead. In the end I bypass the live and switched live to prove the interlock is at fault.
I disassembled the unit and found that the “slug” I will call it that heats up and pushes a bimetallic strip shut for the timed release of the door catch was at fault. I adjusted the set screw so that the arm caused the bimetallic switch to operate. Why has the mechanism become lazy?
Also what are those little “pucks” of metal called the ones that heat up and expand. Should the be energized all the time

thanks

steve