Why is my golden retriever's hotspot not healing or getting bigger?

I recommend to have her examined by the veterinarian again. When pets do not respond to treatment as normal, it can mean the correction condition has been diagnosed. Those spots on Lexy could be several things including allergies to fleas, allergies to something in the environment, food allergy or endocrine disease. Each one of these problems is detected and addressed differently and I recommend to discuss with your veterinarian which one to start hunting down first. Parasites are often ruled out with an oral medication like Simparica. Food allergy is best ruled out with a diet trial using a hydrolyzed protein. Endocrine diseases are detected with blood work. Environmental allergies are a little more difficult to detect but a response to therapy or a blood test can give an indication. Once you figure out and address the underlying cause, the lesions should not come back. There are a couple things you can do at home and they can help but they are unlikely to address the underlying cause. You can try adding in an omega 3 fatty acid supplement for dogs, this can help improve the skin health. You may also try changing her diet to something like Royal Canin or Nutro as they are aimed at skin health. Also protect the area from trauma by having Lexy wear an elizabethan collar at all times. Best of luck and please feel free to post any additional questions.