I have experienced this several times so I feel your pain. Georgia(my youngest) had a flea allergy which caused her to dig bald spots and had little pimple like bumps all over her belly. I could never find fleas and I treated her every month but the problem was the flea stuff I was using wasn’t killing the eggs. I finally bought Advantage and within a few days she had stopped chewing and scratching and her fur was growing back. Then we got Lady. Lady was abused and came from a poor environment and had contracted a mite that only Pitbulls get(this was news to me!). Both dogs were losing their fur! I took them to the vet where he just looked at them and diagnosed them with this and gave them medicine (65 dollars per dog!) Two treatments and they were back to normal. Another suggestion could be something you are using to clean with. Both my girls are very sensitive to sprays, candles, carpet freshener etc. I have had to switch to making my own cleaning products (which is a lot cheaper!) and so far we have went almost a year with no bald spots, chewing or digging. We also treat our yard once a month for fleas, ticks and mosquitoes because Georgia is allergic to those bites as well. Good luck!