What I would do is work with an appropriate treatment for symptoms which most closely match some poultry diseases. This link may help you identify the possibility:
http://edis.ifas.ufl.edu/ps044
If a hen dies, tightly bag the bird and refrigerate it if you cannot get the bird to a lab for a...
You seem to be doing a good job with feed and supplementation. The problem with ivermectin's effectiveness is that mites reproduce faster than they can die from ingesting blood containing the ivermectin. If they are in the coop then they just re-infest the bird once the ivermectin is not present...
Ivomec won't do anything for worms, lice, or mites. I've tried it and it is worthless. Safe-guard liquid labeled for goats gets rid of all worms but tapeworms. Valbazen, Wormout, Wormazole are a few others that get rid of all poultry worms. You used valbazen, and it is a safe, very effective...
Bogtown chick,
What you could be dealing with is a form bacterial enteritis or intestinal damage due to intestinal worms or coccidiosis protozoa. Garlic is a good supplement for the immune system but will not prevent nor cure coccidiosis, and there is no scientific proof to support those...