If the resin is still soft or gummy after 24 hours, the typical reasons are:
1. Temperature is too low.
2. Improper measuring of components.
3. Mixing of components not thorough enough.
In the first case, its possible the epoxy simply requires more time to cure. If possible, increase the temperature to help expedite this process. In the second and third case, unfortunately, the epoxy will need to be removed completely to start again. There is no liquid thinner or hardener that can be put on the surface to harden it further. Use a scraper and lacquer thinner to remove uncured epoxy.