A gel pack is mostly water; a thickener such as hydroxyethyl cellulose or sodium polyacrylate turns it into a gel that does not flow. In a freezer this pack freezes as hard as ice, and its cooling comes from the heat the water absorbs as it melts.
Some packs have additives that keep the water from solidifying. These come out of the freezer as a thick, pliable gel that can be wrapped around the product.
If a pack that normally goes rock hard still has a soft patch, it has not finished freezing. Leave gaps between packs stacked in the freezer and give them more time; set aside any pack that swells, separates or leaks.
