There are many factors that can contribute to mold rubber not curing.
The most common reason for mold rubber not curing is inadequate mixing of Parts A & B. People mixing rubber for the first time tend to under-mix the material before applying. It is especially easy to under-mix thick materials like silicone or brush-on rubber. Mixing large volumes of material also poses the risk of inadequate mixing.
Solution: We have an in-depth guide about mixing process that you can read here – FAQ: What is the best method for mixing mold rubber?
Cold Material or Work Area
Our mold rubbers are room-temperature cure (or RTV), meaning that 73°F (23°C) is sufficient heat for the curing process. Often, when mold rubber does not cure, it is because the core temperature of the liquid Parts A & B is too low, the work area is too cold, or the model itself is too cold.
Silicone and urethane rubber can be stored at low temperatures (even freezing), but must be brought up to room temperature before use. For more information, refer to the article – FAQ: What are the effects of cold weather on urethanes or silicones?
Working in the cold or over a frozen model? Try an accelerated tin-cure silicone. In-depth information about working with these materials in the cold can be found in the article – FAQ: What effect does temperature have on the mold making and casting processes?
Cure inhibition occurs when a surface contaminate prevents a material from curing as expected, and is typically diagnosed by having a rubber mold that cured correctly on the top or outside layer, but is gummy or uncured against the model surface. Typical sources of cure inhibition and remedies can be found in the guide – FAQ: What is "cure inhibition"?