It would help to see a pic of the nests or nest box you have. It could be a matter of redesigning them. Also I would put some wooden eggs in the boxes and leave them there. Even though you see cute pics of nests these are not the best nor advisable.
So your nest boxes should have an opening big enough for them to get into but the nest box itself should not be so big they can stand and turn around in it. It should also be dark. " however in addition to a decent height tip, it s also worth fitting a loose curtain to the front of the nest box, which hangs down to cover most of the opening; hang it from the top but leave a gap down one (sic) otherwise you'll deter the birds from entering" PP mag.
If you feed them egg shells they should be crushed to such a size they are unrecognizable to the birds. I mix the crushed shell in to the grit.