If your pet always seems to be scratching and you can't see evidence of fleas (black droppings over the skin) then sorry but there is a good chance that it has some form of alergic reaction to the food you are feeding - check the ingredients list and look out especially for wheat (used as a cheap filler and generally ends up as more poo).
It is also quite possible that your pet may have an allergy to one or other of the ingredients that would not be immediately considered as a problem food (eg Chicken, Rice, Maize) - the Burns range of foods include varieties based on Chicken, Duck, Lamb, Pork or Fish and the Pork is paired with potato rather than rice. It is well worth trying one or other of these.
In fact all of our foods are Hypo-Allergenic and have been chosen for the quality of ingredients so you can try a few varieties to check for allergiuc reactions. The process of isolating any particular irritant can be quite time consuming because you need to ensure that all toxins have a chance to be removed from the body before you can make a final decision. It also means that you should remove all extra tid-bits you may be feeding as well as the food.
We would seriously suggest the Burns Chcken & Rice as the first try because we have had a large number of positive outcomes once clients have switched to this food.