Reader Zavia wants to know why corstarch-thickened soup sometimes thins out as it cools. The answer is that it depends on what’s in the soup. If there’s much fat, it could have prevented the cornstarch granules from absorbing enough water to begin with. Which is to say, the starch granules didn’t get a chance to fully swell, and then released the little bit of water they had when the mixture cooled. The result: thinning. Try stirring in a little more water into the re-heated soup, and see if that helps it thicken. If it works add a little more water until the soup is back to the texture you want.
Liquids that are very sweet can have the same problem: sugar molecules prevent the cornstarch granules from absorbing the water they need, so when the mixture cools it thins. Again, adding a little more water to the re-heated mixture will usually help it thicken up, as odd as that sounds.
Sometimes this trick won’t work, for example if the mixture is very acidic, as the acid completely breaks apart the swelled starch granules as the mixture sits. For more on how starch thickening happens, and sometimes “un-happnes” go here. I should add freezing completely and irretrievably thins out cornstarch-thickened liquids, because ice crystals expand and break apart the starch granules that contain them. In that instance you’ll need to warm the mixture and add more starch.
I hope this helps, Zavia! Good luck with the soup!