A year ago I bought a tube of Silicone II caulk for use with a caulk
gun. I found I didn't need it. It sat unopened in my house until a few
days ago.

I had a pair of athletic shoes whose heels caved in after three days of
walking. The space under the inner sole was mostly voids. I figured I
could fix the shoes by filling the voids with Silicone.

Apparently the Silicone had gone bad. It took a lot of pressure to pump
it out of the tube, and in three days it hasn't cured.

Is that a common problem? Is there anything I can do to cure it?

The tube says GE will replace the Silicone if I send them the tube and
proof of purchase. That sounds like more trouble than it's worth.