Skin allergy on dogs

My  American Bulldog has been scratching for a month. No fleas, no ticks cant find anything. Can anyone help?

My first bet would be an allergy to his food. Go check out the ingredients on his food. They are listed in the order of greatest to least in regards to the amount in the dog food. Even some of the supposedly vet reccomended and sold ones the first ingredient is often corn. Some other bad stuff is soybeanmeal and corn gluten. most dog foods have some or all of these but you don't want to see it listed as one of the first few items on the ingredients. I recently tried to switch dog food to a cheaper one. They both have lamb as their main ingredient and the only big difference is that one has corngluten and the new one has soybean. Both are cheap protein sources. My dogs immediatly started SMELLING. I have never had a bad "dog smell" with any of my current dogs until I tried to switch. I had great success using a product called "bits" for a dog I had once that would just chew her hair of and get terrible hot spots. I can't find it now but I think they just changed the name and this is the same stuff.
http://www.gardensalive.com/product.asp?pn=1926

Leah thanks for the info I will give it a try along with trying new food,

let us know how it turns out so we can add to our collective knowlege!