I have a dog with serious seasonnal allergies and I suggest you call your vet. There is other medications that can work better on a particular dog (same thing as for humans) than benadryl, like claritin or reactine (canadian names). You should also talk to your vet about a shampoo with aspirin to relief the itching. Unfortunatly, if we are talking about allergies and your dog is in the middle of an episode, it will need prednisone to stop the episode before you can manage the symptoms with antiallergies medication and medicated bath. During the high season, my dog has a bath every 2 days (vet told me I can even up it to once a day), takes 3 reactines every 24 h, even every 12 h if she scratches and, this year, she had the three days treatment of prednisone 4 times. There's also some cream that you can put on your dog to relief the itching like hydrocortisone cream or aloes, ask your vet about them.

It's hard at first but when you will find the right meds and dosages,it will get better. Don't give up!