Doves are not the same as Pigeons. They have no homing instincts at all...none what so ever. If they return to the loft....is only by chance. Increasing your nest boxes will not help. If a pair of Doves are raising young they should not be allowed "free-flight". It takes both parents to raise the young. If something did happen to one parent, while out....you risk the health of the babies and the one remaining parent.
Again.....Doves do not home!!!! The "Dove Release" you see at weddings/funerals are NOT Doves. They are white homing Pigeons that have gone through extensive training.
Doves are easy prey to all kinds of predators when they are out flying. They have no natural instinct to run for cover/home. Please do more research about Doves. It would be really sad for for you to lose these wonderful birds.