Susan, the adult birds bash the mealworms on something hard to kill them before feeding them to very young chicks. Mealworms will bite when a bird is trying to swallow them and the youngest can’t handle that. When they get older there is no need to kill the mealworms. YOU don’t need to kill them. The birds will handle that when necessary.
You have seen fledglings hanging around? That’s good because maybe they will help feed the ones in the nest. Sometimes that happens. How many are in there? I know you said 3 in the first nest but I haven’t been able to find where/if you said how many this time.
I hope she finds the mealworms you offer soon. It can take awhile for some birds to make the discovery no matter what you do. The method Carol described should work. If it does, be ready! That female bird will quickly become hooked.