If you are not sure what is causing the skin problems it is going to be hard to recommend a different food though I do think changing to another food would be a good idea. It may seem like you are getting a better deal with Pedigree, but in the long run you are paying more because you have to feed more of the Pedigree to meet their nutritional needs.
Unfortuntely you will need to have testing done to see what they are allergic to. Could be something in the environment, could be something in the food.
Look into Fromm Family Classics Mature. Much better quality food and you can get it online for a little over a 1$/lb for a 33 lb bag.
http://www.chewy.com/dog/fromm-family-classics-mature-formula/dp/32644
