My cat suffers with flea allergy dermatitis, we think! I’ve been having to apply Advocate spot on to her monthly to help with this. I don’t like doing that but as I’ve never regularly flea treated my cats but she scatches until she breaks the skin and it bleeds and is difficult to heal with the scatch itch going on. If I didn’t use the spot on regularly she would have to have a comfy collar on and I would use one the antiseptic powders you can buy over the counter. However this last summer, after 2 weeks, i.e. before she was due the spot on, she was scratching these pimply things on her skin again and breaking sores again. So I took her back to the vet who said, there’s so many insects about it could still be that, not that she has fleas but they could be irritating her, especially if she’s a hunter (which she is), or it could be some other allergy as well, such as to pollens which when they breath in can irritate and come out in the skin. Anyhow she gave me a steroid cream for her which you only need the tiniest amount and I apply thinly and infrequently, if I see her scratching and starting to break the skin. It does seem to help, and I don’t use it too often, it’s called Isaderm. I hope you manage to sort your cat’s though because it can be a pain.
Edited just to add, the irritation with our cat is usually round the head/neck area. Although when we first adopted her (from another home nearby) her fur was all growing back all over her body, and they said she’d been told she has a flea allergy.