Hi, I have been refinishing floors exclusively for several years so my experience level is quite high with 50-75 houses per year. Over the past 2-3 months, I've noticed the first coat of polyurethane taking longer and longer to dry on water popped floors. I use Lenmar semi-gloss for first and second coats. Third coat is up to the customer. This past Wednesday, I stained 1,200SF of 60 year old White Oak with 1 part Espresso: 1 part natural stain. I let the floor dry two days after staining before first coat of poly and usually I have been able to reapply a second coat in 24 hours. Today was 48 hours and still not ready. This has been getting longer and longer like I said the past couple months and this is the worst. I finish sand with 120 screen before water popping for additional info. Second and third coats are drying in typical 6-8 hours. This leaves me to believe that Minwax has changed something in their formula lately which is not drying up like it used to. My first coat coverage rate has not changed either as is nothing else has changed except we are heading into cooler temps which is usually a little slower but nothing like this. I also only use quarts, not gallons of stain. Anyone else noticing the same results?

With water-popped surface you absorb lost of stain and solvent so it will take longer than not popping. A heavier pigmented stain kind of closes the grain at the surface so this will take longer to harden. Oils don't really dry, they harden. The solvent dries, but not the oil, it hardens. It's a chemical reaction that depends on warmth and oxygen. Using a fan to keep the warm air from rising to the ceiling and dispersing the solvent will help a lot. I prefer Duraseal stains, made by Minwax for floors, but the Quick Coat stains could be what you need for faster dry times.
Of course the lower VOC finishes usually take more time between coats. The quart cans of finish are usually the old formula which had more reliable dry times.
Finish and stain manufacturers sometimes change their formulas which can change dry times without any notice, however. A current MSDS sheet may be different than a previous one.