How old are they and how long have they been outside? Sometimes it takes them a while to try a new type of food. It may be that all you need is some patience.
Certain things like earthworms, slugs, or grasshoppers can transmit certain parasites to the chickens if that parasite is in your area to start with. I could give you TB if I have it and you don't, but since I don't have it I cannot give you TB. Same thing. If those parasites are present the chickens are going to find them anyway, whether you feed them any or not. If it is there they will get it no matter what I do so I don't worry about it. I feed them what I get out of the garden and let them find what they can.
I butcher chickens all the time. When I do I cut the intestines open to look for worms. I have not found any yet and mine eat earthworms, slugs, and grasshoppers.