Need Help with Silicone Rubber NOT CURING

Assuming you got the AB proportions correct I think your problem is how old the product is. The shelf life for silicone mold rubber is only 6 months according the the Alumilite website. I've gone beyond that shelf life plenty of times without issues, but 3 years is pushing it a bit.

I use the same stuff and provided you had the proportions of part A and B the same as shown in the instructions than I would say it was a problem with the batch, you could try heating the molds up in the oven at a safe temp that won't melt the heads like 130 degree's because heat speeds up chemical reactions(I sometimes do this with my molds when I don't want to wait a day for them to cure). If that doesn't work then I would clean the heads with hot water that has lots of soap in it, and if the heads are from your fodder box and are going to be repainted anyway I would add a small amount of finger nail polish remover to the mix(which may strip the paint but will also help with cleaning them).