if the collars coming off, its way too loose, remember a fert has fur, therefore neck looks thicker than it is, my mk3 collars have holes to make the collar smaller than the ones already in it from new,
put the collar on, pull it tight around neck, at this point, its still too loose as its fur underneath, so mark the hole at least another 5mm along, if the fert goes blue, its too tight,
put the collars on them each day in the cage, get them used to wearing them, if they keep getting them off, make more holes and do it tighter the next time, untill the wear them all day, far better t