Hi Brahmamama2010,
I'm sure you're doing everything right. These things just happen sometimes (for instance some birds have extra requirements for nutrients; or sometimes feed may be slightly older when purchased). Really there are just millions of ways feed can be a little low in nutrients, so it's not something to blame yourself for.
Gail Damerow says to toss a handful of dry cat food in the breeder pens for 2 weeks prior to collecting eggs, to make sure they're receiving the best nutrition. I don't know about this (have never tried it), but I know you can get soluble vitamins in various forms. However the easy way to solve this is to buy a specialist breeder feed. Specialist breeder feeds have extra vitamins and minerals added to ensure good eggs.
I never see perosis in chicks I hatch now. I'm not sure what it was related to in the past, but I do find that fresh food (eg. I mix my own sprouts, soured milk, protein meal, legumes and seaweed) seems to keep it from happening. However not many people could go to this trouble, hence the suggestion for a breeder feed.
Meanwhile you might google perosis or slipped tendon, so you can see for yourself if it sounds like the case with yours.
good luck
Erica