I'm afraid my adhesive sticks tend to dry out over time. I dare say I quite often don't close them well enough. Does anybody know of a way to revive them when this has happens, or are they absolute gonners?
Dip a cloth in warm water, squeeze out excess water and place it over the tip of the glue stick and close the cap over it. Glue sticks generally consists of 40% water and 40% Acrylic polymer (Referred from Wikipedia), so re-moistening should help. Since it is a polymer, a higher temperature will help in speeding up the bonding process of water to the components in the glue stick. Make sure you don't over moisten it with too much water, which may result in the loss of adhesive property of the glue. And also make sure you change the cloth periodically so that it doesn't dry up and stick to the glue stick.
Add water in the cap and let the glue stick sit a few days, with the cap on top.