Itching dogs can be very frustrating. I would encourage you to have your vet perform a few tests to get to the bottom of this problem.
1-Skin scrape to check for mange.
2-Fungal culture to check for ringworm.
3-Culture and sensitivity to find out what is growing on the skin and what medication is effective.
If these tests are negative, you may need to run some bloodwork to check for underlying diseases such as underactive thyroid or cushing's disease, both of which can lead to skin problems.
Once all these tests are run, and the conditions mentioned have been ruled out, then we are left with the likely diagnosis of allergies. Allergies are diagnoses of exclusion. That means that we have to rule everything else out, then we can call it an allergy. Once we determine that an allergy is the likely diagnosis, then we have to determine what kind of allergy we are dealing with.
In Florida, and in many other parts of the country, flea allergies are very common. You may live somewhere like Colorado (I don't know) that doesn't have fleas, but I will mention this anyways. The most effective treatment for flea allergy dermatitis (FAD) is to use a quality flea preventative. I usually recommend advantage or comfortis. You may even need to use it more frequently than it says on the label to assure complete flea prevention. Every 4 weeks is the norm, but you may need to go to every 3 or even every 2 weeks for adequate control depending on the environment. Fleas are an environmental problem. For every adult flea on the pet, there are 100 eggs and immature larva in the environment. Both inside and outside the house need to be treated, and repeated regularly. Flea larva love carpets, dog beds, cracks in wood laminate, sofas, kid's bed, and the yard among other places. You may need a professional to get involved. Fleas are very good at what they do so regular treatment of both inside and outside the house is necessary to control the overall population of fleas in the environment.
Another type of very common allergy is atopy, or topical allergy. This is when the dog is allergic to environmental allergens such as dust mites, grasses, pollens, molds and other microscopic allergens. Treatment for this type of allergy involves frequent baths (which you are doing) up to 3x per week. A daily fish oil supplement is also recommended for life. Omega-3 (fish oil) has a natural anti-inflammatory effect and concentrates in skin for dogs. A daily supplement will help reduce the need for anti-itch medication and can reduce the clinical signs.
Antihistamines are also very useful. If the benadryl helps, then continue to use it. You can give up to 1 mg per pound every 8 hours as needed for comfort. It can cause sedation, but generally is very safe for use in all pets.
Another type of allergy is food allergy. Dogs can develop allergies to different components of their food, and it is difficult to determine which component is the guilty party. There is no good lab test to figure this out. The only way to do so is to run a food trial. This means putting the pet on a prescription hypoallergenic diet for a minimum of 8-10 weeks. The important thing to understand is that nothing else can go in the pet's mouth for the entire 8-10 weeks, or you are back at the starting point. The way the body responds to food allergens, it takes a long time for the body to clear them. Anything, and I mean, ANYTHING that goes in the pet's mouth is suspect. Even chewies, pigs ears, and monthly heartworm preventatives have to be considered as suspect. Once the trial has gone on for the minimum testing period, if the pet is doing better, I will sometimes suggest challenging them with their old food and see how they do. Or they can just stay on the hypoallergenic diet if they are doing well. Owner's choice.
A couple of other types of skin reactions can occur as well. Contact dermatitis (especially on the paws and belly) if there are caustic agents or plants in the area. Stings from insects and spiders can cause reactions as well. There are also various types of skin cancers that can occur. Usually biopsy is required at that point to come to a diagnosis.
One other point I would like to make. If the pet needs to be on oral meds for bacterial or fungal infection, usually a minimum of 4-6 weeks is needed to clear infections of the skin. Sometimes even up to 16 weeks in deep infections.
She may need to be put on steroids for a period of time to help control the itch until the appropriate treatment starts working to reduce the inflammation.
good luck and I hope this helps.