lovebird feathers missing

Question
I have a small colony of six pairs of p/f lovebirds breeding, chicks from two box's have flown the nest looking perfect but chicks from two other box's have popped out with no main wing feathers and unable to fly, in one box fully feathered except the main wing feathers the other box the chicks have more feathers missing, the adults have produced young in previous years no problem, they are fed a lovebird seed mixture with a regular supply of fruit and veg as well as egg food (dry), can you tell me whats wrong and will their main flight feathers grow, the chicks are eight to nine weeks old, hope you can help,many thanks.
Neil

Answer
Normally the feathers will eventually grow in.  Some parents get the habit of plucking the feathers of their offspring and it is hard to cure it.

Some reasonable theories say that vitamin deficiencies in the parents lead to the initial behaviour which then becomes a habit.

With parents like these the best thing in future is to remove their chicks at about 14 days and hand feed them.