It did not cure at all
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Silicone: inhibition
Certain materials stop silicone curing where they touch it: sulfur-bearing clays, latex, some adhesives, some uncured resins. The bulk cures and the contact surface stays liquid, which ruins the surface that mattered. Platinum systems are much more prone to it than tin.
Which is why you test first
Curing a small amount against the master before committing the full pour costs minutes and catches this before it destroys both the silicone and the model. It is standard practice and routinely skipped.
Resin: too cold
Below the product’s minimum temperature the reaction can stall entirely. An unheated garage in winter is the usual setting. Warming the components and the space before mixing is the fix, and warming only the mixed resin is not enough.
Or a part was never added
It sounds unlikely and it happens, usually when something interrupts a pour. A liquid that never changed at all, rather than one that thickened and stopped, points this way.
Or the material is old
Resin and especially hardener have a shelf life, and hardener that has been open and exposed to air can lose activity. Date your containers when you open them, which costs a pen stroke.
Clean up as uncured chemical
A pour that never cured is liquid resin, not a solid object. It is waste with rules attached and it is an exposure while you deal with it, so gloves and ventilation apply to the cleanup as much as to the pour.
Save the failed piece until you know why
It is the evidence. A liquid that never thickened, one that gelled and stopped, and one that cured everywhere except one face are three different diagnoses, and throwing it away before looking properly loses the only record of which it was.