Thickening old mushroom soup
For cornstarch to thicken, two things are required: a temperature of 95°C/203°F, and to be left alone for a while to gather its thoughts. Cornstarch causes thickening by building a network of starch molecules. Once this has formed, stirring just breaks the network and the thickening effect is ruined.

Make up a new cornstarch slurry and add it a little at a time to the simmering soup. Only stir it enough to incorporate it through - don't keep stirring after that.