Have you ever tried givind them Mastomys natalensis (don't know the english name) instead of ordinary mice.

Picture:

source: Veeltepeltmuis

They have a different scent and usually work very good on kings. But beware, never give them alive, they are usually very aggressive. It's like they miss a screw or 2 ( 3, 4, 5 ... ).You also can't handle them by hand... Such nice animals, I used to breed them awhile