I've got some popcorn that I've had for a while, and when I try to pop it, a lot of the kernels remain unpopped. Is there anything I can do to revive it, or does it go in the compost?
I store my popcorn in the freezer in an old mayo jar with the lid tightly screwed on. As Michael says, dud popcorn has lost moisture. Add a tablespoon or so of water to the jar and put in the fridge (not the freezer for this part) a few days and see if that helps.
Storing uncovered in the fridge or freezer will remove moisture if you have a frost-free refrigerator.
The reason it has stopped popping is that the kernels have lost some of the moisture inside that provides the explosion which pops the shell. If you store them uncovered in the refrigerator for a few days, they will absorb moisture and start working much better again.