Why does hard candy turn sticky?
A properly made hard-crack candy didn't fail in the pot -- it's reacting to the air around it after the fact.
The kitchen answer
Hard candy going soft and sticky in storage isn't spoilage, and it isn't a sign it was undercooked. Sugar itself actively pulls moisture out of the air, and unsealed candy in humid conditions simply reabsorbs the water that was boiled out of it.
What's actually happening
Sugar is hygroscopic -- it has a strong chemical affinity for water and will absorb ambient moisture from the air given the chance. Hard-crack candy (see Sugar Work) is cooked down to roughly 99% sugar concentration precisely because almost no water is left to plasticize it, which is what gives it a glassy, brittle texture. That state is a temporary snapshot, not a permanent one: left exposed to humid air, the same hygroscopic property that made the syrup concentrate work in the first place starts pulling atmospheric water back in.
As moisture is reabsorbed at the surface, the sugar concentration there drops back down, and the surface softens and turns sticky -- the exact same water/concentration relationship from the candy thermometer, just running in reverse, over days instead of minutes on the stove.
What does that mean for your recipe
Airtight storage isn't optional for hard candy the way it might be for other bakes -- it's directly counteracting an active chemical property of the finished product, not just keeping it "fresh" in a general sense. Wrapping individual pieces (rather than storing a whole batch loose in one container) also limits how much surface area is exposed to air at once, since it's a surface-level absorption effect.
What can go wrong
- Candy made and stored on a humid day: reabsorbs moisture faster from the moment it's cooled, even before proper storage begins.
- Loose storage in one large container: more shared surface area exposed to whatever air is inside the container, compared to individually wrapped pieces.
- Container opened frequently: each opening refreshes the air inside with new ambient humidity.
- Blaming undercooking: a candy that hit the correct hard-crack temperature and still turns sticky in storage is a humidity problem, not evidence it needed more time on the stove.
Try it yourself
Same batch, two storage conditions:
- Make one batch of hard-crack candy, divide it in two.
- Store one half in an airtight container.
- Leave the other half loosely covered, exposed to normal room air.
Watch for: the exposed half visibly softening and turning tacky within a day or two (faster in humid weather), while the airtight half stays glassy and brittle for far longer -- direct proof the change is about moisture exchange, not time or temperature alone.
Go deeper
Hygroscopicity isn't unique to hard candy -- it's the same underlying sugar-water affinity behind why sugar helps baked goods stay moist for longer (sugar competing for and holding onto water molecules), just working against you here because there's almost no water left in hard-crack candy for the sugar to hold onto instead of pulling more in from outside.