Resin cures by heat.  When you mix up resin, you should always immediately pour it onto your work.  If you leave a mass of mixed resin in a container, it will build up heat rapidly and dramatically reduces working time.

If the resin feels like it is getting very warm, you should not try to apply it.  Once it gets to this point, in a matter of seconds, it can change from very warm to very hot and start to gel or clump.  It is best to discard the resin and make a new mix.