silicone moulds should all be post cured for a few hours at around 400 degrees to assist with the
"silicone matrix cross-linking", and also to assist in compression setting.
the silicone will still shrink up to 4% after it has been initially cured, and when a two part plastic starts to cure it releases a lot off heat that could force a post cure on the inner portion of the mould causing both sections to bond or fuse, specially if the release has not been renewed each time.. or is silicone based itself.
another issue can be humidity or water in the components of the plastic, too much of one part over the other or cross contamination and last.... age of material, once opened shelf life degrades exponentially regardless of what it suggests on the label.
just my personal experience...
