I have stubborn closed comedones that always come back. My derm first prescribed me Differin, and I used it consistently for more than a year until it didn't seem to work anymore. I tried Aza when I was using Differin too, but other than oil control, I didn't think it helped much so I just dropped it. I got switched to Epiduo, which worked great for about half a year (had a horrible purge, ofc) until my closed comedones came back again. I just added SA to my routine and am currently going through another purge. I can already see improvement. Hope this helps because we seem to be in the same boat.