This might help you understand how important what they eat is.
On average a full grown hen should eat about 3/4 cup worth of feed per day.
If they are getting 1/4 or even 1/2 cup worth of treats per day do you see how that really takes away from them getting the "whole" balanced meal of 3/4...
"We also give them lots of treats, including a handful of dead worms a day."
This can be part of the problem too.
You wouldn't have to worry about them having any deficiencies if you feed them a balanced diet.
A balanced diet would be a pelleted or crumbled layer type feed ONLY.
Feeding...