It might be seasonal alopecia. I am not sure how likely this condition is for pugs but it can happen to Bulldogs. You may want to try lipiderm capsules (Petco or Petsmart should have them). They basically supplement fatty acids and oils that are great for the skin and coat. In many cases using the lipiderm capsules can speed up hair regrowth.

While the capsules can't really hurt, I would still check with your vet about the condition of your pug.