feather problems

i got a non plucking feather problem with my 4 year old female princess of wales parakeet she is half latina half normal basically the feather grown in as stalks mostly in the primaries the secondaries never grow in the tail dosent grow when feathers do grow they never comeout of the sheaths she is caged with a male of same breed who is in perfect condition i give her liquid iodine in her water which seems to fix the problem but i really need to discover the root cause