I have found two things guaranteed to make my backside and legs go numb: the front of the seat too low, and the sides pressing in on my backside.
More than once I have found that a seat that created "numb bum" stopped doing it if I raised the front of the seat or made the sides a bit wider or a bit of both.
I have even had situations where I while I was paddling my legs had gone numb within about an hour but the numbness went away in just a few minutes after I stuffed something under the front of the seat to raise it about two inches. I didn't even need to get out of the kayak to fix the problem.