Did you mix up a very large amount of the epoxy? If you did, the mixture could have got too hot and increased the cure rate. Curing epoxy is an exothermic reaction e.g. it generates heat as it cures. a normaly small volume can disipate the heat generated to the outside, but a big glob can not get heat out from the center quick enough and the mixes temperature increases. I like to mix my stuff on a plate of glass with a spatula. It helps insure good mixing and the stuff doesn't warm up.