Epoxies require two components—resin and hardener—that, when mixed thoroughly, start a chemical reaction that causes the mixed resin to harden into a plastic solid.
For a proper cure, it's critical to mix the exact ratio of resin to hardener, and to work in the temperature conditions specified by the manufacturer.
A common mistake is to add more hardener, thinking it'll speed up the curing process. Epoxy doesn't work that way—it cures when all the resin molecules cross-link with all the hardener molecules. Adding more hardener will upset the balance, with unlinked hardener molecules that result in an uncured mess or inadequate cure strength.
When working with epoxy, always measure the exact amounts of resin and hardener specified by the manufacturer, and mix thoroughly for a few minutes so the resin and hardener are blended thoroughly.