Silicone mould won't cure at all

I have been attempting to make a fairly small 2 part silicone rubber and fibreglass jacket mould, but every time I mix a batch of silicone it doesn't work. It doesn't even start to cure, it just stays the same consistency as when I started. I'm using T20 silicone, and mixing it with a small amount of thixotropic agent to thicken it, and the last 3 tries nothing has happened. Nobody I've asked can explain why I cannot get my silicone to cure at all, and I don't know how to proceed. Would be grateful for any help.