Bytor,
It's possible some silicone or mold release agent may be reacting with the resin, or possibly the proportions of par A and B might be off.
I've found I have fewer problems if I follow these guidelines:
1. Make sure you carefully mix the correct proportions of resin part A and part B. I use 3ml syringes.
2. Ensure you completely mix the two parts before you fill the mold. I use cut-down film canisters and stir with a flat stirrer.
3. Soak the parts in a strong detergent before you use them (some people recommend a bleach used for white wall tires).
4. Use an automotive acrylic primer for the resin.
5. Wait at least a 2-3 days before using the part to allow the resin to completely cure - I don't know if this really makes a difference, but I've found that the resin bits seem "dryer" if they've sat for a week.
Good luck.