
have you had them checked by a vet? its possible they have skin allergies or some form of mite if they are scratching a lot? fleas arent always seen, but generally if you look hard enough you will find some evidence of them, so possible not that.
if frontline isnt effective, you can ask your vet for other products that may be more suited to your dogs? also, have you any cats or other dogs that visit, it could be possible they are bringing them into the house?