Try using special shapooos or go to a vet to make sure it's not likned with cacer or something.
My Pom lost started to lose her hair with age, her whole coat thinned. The places we noticed it most was her neck, and hind legs.
Flea collars in general will eventually burn the hair off underneath then. Try Frontline or Advantage. It will take months for regrowth if it is due to the collar.
I have an older pom (9 years) that I have shaved every summer - hair never came back on his back - I started giving him fish Oil vitamins (from vet for dogs- you can put them in "Greenies" pill pockets to ensure they are eaten) - his hair is growing back now & he seems to be a lot healthier with more energy.