Question about Silicon

I bit the bullet and installed a fan on the back of my coulter reflector. I reinstalled the mirror last night using silicon that is probably a year to 18 months old. It was very hard to squeeze out of the tube, but in my haste to get done I applied it anyways. That was 21 hours ago from the time of this post. It still has not set up properly. It is not sticky but is moldable if squeezed between the index finger and thumb. I haven't removed the supports (stacks of pennies) for fear the mirror will just sink into the hole I cut in the back of the mirror cell. So after all of that, my question is - can silicon get old in the tube and not useful? Should I cut my losses and just go buy a new silicon tube. The work was done at room temperature.

Thanks!

David, you said that you're using "stacks of pennies" as spacers. How thick will these silicone pads be? i.e., how many pennies? Too thick and they will still have some give to them, even with a fresh tube of silicone glue.

Doug