I was thinking flock raiser was a higher protein than layer but maybe not? We don't have flock rations here.Could be heat stress. Also, it sounds to me like they are getting more than their share of hand outs between AM and PM treats. What is their protein %? I believe that calcium is a very small part of the egg shell equation. I'd suggest putting them on a multi vitamin. I also think that your space is quite tight, and that could be part of the issue. do they get to free range? Have access to green vegetation? If not, I recommend turning their run into a deep litter run.
