Is this lettered on..... or brushed in as a background color ??
Like Steve said, sounds like something was contaminated along the way, whether by you, bad prepping or not stirred enough.
I have 1-shot that is over 12 & 15 years old and it's been opened...... and still works fine.
If you haven't contaminated anything, I would then think the can was not properly sealed and although a minute amount of air got in.... the drying agents have been use up and there's nothing left to complete the drying state, therefore it might not ever cure.
I'd let it go another day or two and if it remains sticky.... I'd wipe it off and use a different can.