The temp and humidity will cause epoxy to cure at different rates as has been said. I haven't had problems like you are having but I'm wondering if you might be mixing it in something that might cause you a problem. For instance, I paint for a living and you don't want to mix epoxy or urethane type paints in something that has a waxed surface. The wax coating of the container can stop the crosslink reaction between the chemicals in the material you're mixing.