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Hi there and thanks for your question. This certainly sounds like a frustrating problem!
It sounds to me like there may be some skin infection present especially if there are bumps. If there is a skin infection, most will need a long course of antibiotics like 4-8 weeks worth to go away.
It sounds like your vet has done a skin scraping to rule out demodex which is a skin mite which can cause lesions like this.
Another possibility is that these are areas of ringworm (which is actually a fungal infection instead of a parasite). Your vet can test this by sending some skin flakes from this area for a fungal culture.
Some Daschunds can get pattern baldness. However, usually this is not itchy...it looks like this:
If your vet is totally stumped and the extra things I have mentioned are not the causes, then the next step would be to have a skin biopsy taken to have a pathologist look at. They should be able to tell you what the problem is and how to fix it.
It could certainly be an allergy combined with some infection as allergy alone shouldn't cause the bumps. However, if your dog is on medication for itching( I am assuming that is a steroid like prednisone) it should be doing the trick for allergy.
Ringworm actually needs special anti-fungal medication in order to go away, so won't be treated by just antibiotics alone.
I wish biopsies weren't so expensive, but often in the long run they are the most economical thing to do rather than trying a whole bunch of treatments as guesses.