Don't know why but mine do it also only on both sizes. I always figured the lead wasn't sticking to the hooks. Since I always powder coat mine I make sure to dip the head to where the paint is at least 1/8th up on the hook/keeper. When the paint is cured the head is then tight on the hook.
I fish on the edge and stay out of the middle
99 BassCat Jag w/225 Merc Optimax