Why did my sink drain blockage clear after several weeks?
Happens here fairly frequently.  Stationary tub and washing machine share a drain line.  Tub isn't used much.  Side flow from the washing machine refills the tub trap with dirty water.  Eventually it gets enough crud that the drain slows.   Use the sink, and the first 3-4 inches of water drain slowly, then it flushes the crud out.

In your case, a slow response to the chemicals seems probable.