My dogs keep on scratching?

I pair of Golden Retreivers have been scratching a lot. I have applied powders containing Clorimatazol, twice and even thrice a day. i have given them medicated bath with Lizoral I have also used Candid B ointment. They were on anti-biotic for some time. But nothing seems to work. They keep on scratching in belly area. Am I administrating wrong medicines?