These sores must be caused by something in the environment. I have 5 Jack Russells. 2 of them have begun showing these same sores in the past couple of months. 3 of my youngest dogs eat Blue Buffalo kibble. One is really old (16) and on prescription diet. One of the younger dogs has had several single sores on her back which I've been treating with topical antibiotics. It will dry up, leave a bump and hairless spot. Then another will show up in another place. My 13 year old only eats homemade food...no kibble she can't digest it. She too is now showing these sores. Hers seem to itch more than hurt. Where as the other dog has pain and itch. Aside from these sores, both dogs are healthy and happy and act like fools, just like terriers do!!
It's interesting that another one of my dogs seems to smell the onset of these sores. She will start to sniff Natasha or Dilly's back, then lick/chew at one spot. When I look closely, I can see a new bump beginning which will turn into a sore and I can start to treat it right away.
Seems like this "sore problem" is a growing problem everywhere from what I have found doing research on the web. If I find anything new, I will post it.
Hope you have found something to work for you.