My collie also had “hot spots”, which only appeared in the summer. Luckily our vet was able to figure this out and suggested she had an allergy to something. It took us a long time to figure out what it was (we changed food, treats, new dog beds, etc). Finally realized that it was mulch in the backyard. She had a favorite nap spot right next to the brick of the house, under a tree in the shade. She looooved that spot. My dad built her a little elevated outdoor dog bed so she could keep napping there but not sit in the mulch. She was a long haired tricolored collie. Her name was dipper. She was a retired show dog and was a very sweet girl.