My Dog’s Skin Is Really Irritated, She’s Always Scratching And Biting At The Skin On Her Back.?

She’s had the problem for a while, and we could never figure out what it was.
First we thought it was fleas but flea treatments didn’t help, and she had it all year round.
Our vet keeps telling me it’s a food allergy, or a reaction to a tick bite, but she’s never actually examined her, so it’s just an assumption, and, though she’s a good vet, she mostly deals with livestock and is responsible for a lot of people’s animals, so it’s like “if it’s not life threatening, don’t bother me with it.”
I don’t blame her for it or anything, because she is busy, but it’s really awful seeing Twix in so much pain and discomfort.
Anyway, she’s part cairn terrier, and someone told me a few weeks ago that they used to have a cairn, and they had similar problems. So I looked it up and it seems that while it could be allergies, it could also be infected hair follicles from dead hair not being cleared away, and I have to groom her with a stripping knife to get rid of it.
Some of the websites had a pretty extreme view on it, and recommended stripping almost all of the dog’s hair, which I really don’t think would be a good idea.
There were a few other conditions which applied specifically to cairns, and were as a result of inbreeding, but I don’t think it could be any of them because she is a mutt, but still…
Does anyone have any experience with things like this? I would appreciate any advice you could give.