I am doing a pumpkin flan, recipe is 325 F for 45 min in a water bath in the oven but it's not cooking at all. What is the problem?

Doesn't sound like you're doing my pumpkin flan recipe ( https://food52.com/recipes/14972-pumpkin-flan) as it bakes at 350 for over an hour - but quantities, etc. may differ. But wondering if maybe a slightly higher temp and longer time might help? Also, yes - the water must be very hot to boiling (I admit I don't always wait for full boil, but definitely close to it.)

Did you bring the water to a boil before you put the water bath in the oven? (When I make a water bath for custard, I put the ramekins in the larger pan and put it in the oven. Then I carefully tip boiling water from my tea kettle into the large pan.) If not, and it's not cooking at all, you may have to change out the water in the water bath. (Water acts as an insulator to prevent the eggs from cooking too quickly, but it takes a long time to heat water up in the oven.) Hope this helps.

Did you put boiling water in the water bath pan? If not, then that's your problem. It will take ages for the water to heat up in the oven.