There are 4 common reasons a dessert that looks thick in the pot becomes watery a few hours later: it was not boiled enough, it was stirred too much after thickening, an acidic fruit was added while it was hot, or it was frozen. All 4 break the network formed by the starch. To fix it, return the mixture to the pot, add a little starch slurry, and bring it to a boil again.
The first and most common reason is not boiling it enough. If the mixture is removed from the heat as soon as it thickens, the starch granules are only partly activated; the resulting network is weak and releases the water it held as it cools. Recipes that contain egg yolks also involve enzymes in the yolks, which speeds up thinning. The solution is simple, but it cannot be applied retroactively: the mixture must be brought to a full boil and kept boiling briefly. It is not enough for it to bubble only around the edges; wait until bubbles appear in the center too.
The second reason is excessive stirring. Once the starch network has formed, continuous whisking mechanically breaks it; the mixture first becomes glossy, then thins. That is why you keep stirring while it comes to a boil, but do not stir the dessert after pouring it into molds. Whisking a cooled custard to thicken it also has the opposite effect: the mixture becomes even more watery. If you need to smooth the surface, do it with a single pass of a spatula, without stirring.
The third reason is acid. When lemon juice, sour cherries, pineapple, or sour fruit puree is added to the hot mixture, it breaks down the starch chains and visibly weakens the texture. For that reason, add acidic ingredients after removing the mixture from the heat, once it has cooled somewhat. If they must be added, increase the amount of starch slightly from the start. Sour fruit juice poured into the hot mixture can immediately ruin its texture. For the same reason, add fruit pieces after cooking, placing them between the layers as you pour the dessert into its mold.
The fourth reason is freezing. When a dessert thickened with cornstarch is frozen and thawed, water separates and a clear liquid pools on top of the cream; the texture also becomes grainy. This separation cannot be reversed. That is why starch-thickened desserts should not be frozen and should be eaten within a few days when kept in the refrigerator. For recipes that need to be frozen, choose a different thickener. Waxy corn starch and tapioca starch withstand freezing better than regular cornstarch. Even if you blend a thawed dessert, it will not return to its original texture; it will only hold together for a while.
A watery dessert can often be rescued. Return the mixture to the pot; in a separate bowl, dissolve a small amount of starch in cold milk, then add it gradually while the mixture is boiling. Continue boiling for 1–2 minutes, then pour the dessert into the molds again. Do not sprinkle starch directly into the hot mixture, because it clumps immediately and those lumps will not break up. Reboiling the dessert changes its flavor somewhat, so make this adjustment on the same day.