3 of my 6 dogs chew on their feet. I did blood testing for food and environmental on one of them. What is your belief on the blood tests for allergies?

I have 6 shepherds and live in the country. I have an outside allergy/food allergy question for you and am looking for some advice. 3 of my 6 dogs chew on their feet. I did blood testing for food and environmental testing on one of them twice to see if I could get any information to pinpoint the cause. The results were different for both tests, which leads me to believe its less than accurate (first test showed her positive for duck, second test showed negative). What is your belief on the blood tests for allergies, or have you heard of a common denominator for why a dog would chew their feet? I'm wondering if it isn't a contact allergy vs a food one. I am considering doing my own home natural diet as I know you recommend it. I have always fed high quality dry food such as taste of the wild and natural balance, but Id rather get rid of this issue completely if you have any advise on what may be causing this? Not sure if its related, but I do use hay outside my kennels that have dirt to keep them from becoming a muddy mess. Still 3 of them have no issues with their feet, but I wondered if the hay itself might be the irritant.

Shellie