The curing of a polyurethane plastic is a chemical reaction in the course of which heat is released. Silicones make very poor heat conductors. Because of this the temperature can significantly increase whereby the mould material can become brittle and break off. This is irreparable. The only option in that case is to cast a new mould.
The service life of a silicone mould is between 20 to 30 castings. In addition, a mould ages naturally (hardens), and after about 3 months, no longer delivers a high-quality product.