Sulfur-based clays, polyester resin, Bondo, and many other materials, known and unknown, can inhibit the cure of Platinum Silicone (PlatSil®) rubber. Always do a small test cure to be sure the rubber will cure. Many shops will keep a small kit of a fast-curing platinum silicone on hand just for inhibition testing.
New Tin Silicone (TinSil®) molds can inhibit the cure of polyurethane plastics and rubber. This is caused by the alcohol vapor that comes off the mold in the first few days of curing. To speed up this off-gassing, place the mold in an oven at 110°F for a day.