Why did my chowder turn out watery?
Unfortunately, cornstarch does that. It does not reheat well. Potato starch is even worse, and that's the thickener in Knorr's Leek Soup. 

If you want to reheat something thickened, your best bet is to thicken it with a roux. There are other more modern thickeners (think molecular gastronomy), but I don't know much about those.