With polyester resin, the amount of hardener determines how fast it sets. The hardener works kind of like a catalyst bringing about a change in the resin.

With epoxy resin, you have to get the mixture right. It's two substances that react with each other and use each other up to produce a third, solid, substance. If you don't get the mixture right one of the two liquids will not be completely consumed. If it doesn't set within the pot life listed by the product, remove it, thoroughly clean the mating surfaces, and try again with the correct mixture.