Though a few posters have commented on your cleaning regimen, and though I'm not entirely clear if your phrase "disc doctor brushes and cleaning fluids" implies that your cleaning fluids are DD brand as well, I'd focus on that as a possible culprit. Hopefully I won't turn this thread into another referendum on the virtues/problems of various fluids, but I would suggest trying a few other brands.
When I bought my used VPI 16.5 on this site, the seller included some DD brushes and DD fluids with the sale. I diluted the DD fluid as specified and also used a distilled water rinse (sometimes up to three such rinses) after using the fluid. I found that I had some improvement, though I also often had pops and clicks, and often this was the case on fairly new LPs that had played much better before I cleaned them. Like you, I was frustrated that the cleaning wasn't doing as much as I expected it to, so I began using two other varieties of cleaning fluids quite popular in this forum. Both of these fluids are much less "sudsy" than I found the DD fluids, and neither requires the use of distilled water as a rinse. No offense to those who prefer the DD fluids, but my vinyl has never sounded better to me. (There are plenty of past threads about various cleaning fluids and theories about distilled water, etc., if you check the archives.)
Hope this helps.
Joshua