I have heard if your egg shells are dirty(poo) on them you should worm them. (The chickens) mine were and I added DE and about a 2 weeks later I did it again egg shells were clean and have been ever since. If shells come out dirty I give DE dry with food pellets and in a few days it clears up I stop the DE, It has worked for us... Our chickens have really stopped laying so we thinned out flock down to 10 girls. Egg production started to improve, today we got nine eggs its still very cold here in WY and we have some sunshine so it all helped.
My girls we thinned were old, we are getting new ones this spring. Just my 2 cents...... Lori