Dick in Reno

We have a 6 month old Wheaten who scratches all the time to the point of distracting himself from training and going outside for toilet training. The vet says he can't see any skins dryness or other problems. The groomer gives him a non allegric bath and also doesn't see any problem. What is going on....we have even tried a half of a benedryl to see if that would work. Needless to say this problem makes it hard to train him and his attention span is somewhat short.. Thanks for any input