Silicone just isn't curing...

[Evil Eye]

Against my better judgement, I decided it would be a good idea to try getting back into casting using silicone (of my own designs and use, before anyone asks). Notably I had the idea that I could design models with transparent elements (canopies, crystals, xenomorph-style domes etc) and print the whole lot in opaque resin, then polish and make moulds of the parts I want clear and cast them in clear epoxy.

To that end I bought some Siraya Tech "Defiant" Silicone; a very well-reviewed platinum-cure silicone (mixed in a simple 1:1 ratio) that works with 3D resin. Before I comitted to casting anything printed though I thought I'd practice with a plastic WHFB forest goblin and make some simple cut-open cup moulds to practice. Unfortunately, I've been having difficulties with the moulds; they cure most of the way, but I'm now on my second mould that nearly 24 hours later still has some uncured, runny bits.

 

Given that this stuff (according to reviews) can't even be cure-inhibited by usually difficult to mould materials like UV resin, I suspect the problem is improper mixing. If it IS just this, I might have a solution (mixing a small amount of pigment into one half, or different colours into each, and then mixing until I get a solid colour to confirm the parts are well and truly combined). However, is there anything else that might be to blame?