I doubt that the wax in the cup was the problem. If proportions were close and the epoxy was mixed well it should have cured. It is doubtful that you would get enough wax migration into the epoxy to cause problems. Wax however is a releasing agent for epoxies and is often used in mold applications to prevent sticking. Any containates on the surface of the areas to bonded like wax, oil, grease, dirt, or dust may prevent a good bond.