Paul is exactly correct on this. I recently started to play with poly resin as it is quite a bit less expensive, but the mix is a little more complicated and I find it harder to work with myself. If you bought your resin at a local hardware or auto store, it is likely the Poly type and the fumes are pretty potent. I actually find MAS to be much more pleasant to work with.

Epoxy hardens based on a chemical process which can be accelerated or slowed based on the amount of hardener. If it is correctly mixed, the surface it is applied to will have little impact on the hardening. The only real impact to your application is a potential adhesion problem. Once the chemical reaction starts, it really cannot be stopped.

The most likely culprit is that you didn't have enough hardener in your mix.