My Princess has allergies & yeast (brown spots on tummy, brown gunk in itchy ears), was on temaril p for a short time. She had numerous skin cytologies done to confirm that it's yeast, it doesn't cost much to get tested (about $30). They do a skin scraping & look under the microscope.
After watching this, I stopped giving her 'sugary' foods. Picked out a grain free kibble w/no potatoes & her brown spots & gunky itchy ears are no more! They were always on GF kibble, but it always had potato or sweet potato in it, also stopped all store bought treats. Now she's eating premade raw food for 1 meal & is doing excellent. The best part, she didn't need any more meds.
Eating These Foods Can Cause Dog Yeast Infection
I would also skip the oatmeal shampoo, dogs that are sensitive to grains might not do so well w/grain based shampoos. You might want to try something w/a coconut base. Princess was also prescribed RX shampoos but it didn't help much. The food change was the most dramatic difference for her. It's hard to keep her away from the grass when we're walking tho.