Mastiffs (and other short-coated breeds) are rather prone to "pimples," officially called pyoderma. It is a bacterial infection of hair follicles. Sometimes allergies can trigger it; sometimes moisture, or excessive skin oils. There are also a few conditions that can mimic it--an unusual form of "ringworm" a type of fungal infection) and some autoimmune diseases (where the body attacks its own cells), but these are rare. If she has not been on at least 3 weeks of antibiotics (usually cephalexin is used, at 10 mg per pound twice a day), I would do that first. It could be that the infection never totally cleared up to begin with. You could also have her tested for a thyroid insufficiency, which can make a dog's skin more prone to pyoderma, and can cause hair loss. If the hair is coming out in patches, she should probably be checked for Demodex, which is a non-contagious type of "mange." If you want to use a medicated shampoo, the best one to use is one that contains benzoyl peroxide, such as Oxydex, Sulfoxydex, or Pyoben.
I hope this helps. Let me know if you have any questions.