With 99% IPA, I usually have to douse the board (or apply with toothbrush) & let it sit for a minute before I start scrubbing with a toothbrush. Then dry it off and FULLY clean the toothbrush with IPA. Now you should have the easy stuff off, so you can re-soak the toothbrush and go at it again, focussing on the built-up areas of flux. Repeat until clean and place in front of a fan to dry. The surface of your joints may look dull, but it won't affect the connectivity. Note that if you need to resolder something after cleaning, you may have to add extra flux to do it.
Some fluxes don't come off easily, even with 99% IPA, so I have moved mostly over to a dedicated spray flux cleaner. I've had good results with
MG Chemical 4140 Flux Remover, but other dedicated sprays should work. It only takes me 2-3 passes with minimal scrubbing to remove even the toughest fluxes.