Why it happens
Stir-frying works by cooking small pieces of food very quickly over a very high heat, so moisture evaporates instantly and the food sears. If the pan or wok isn't hot enough, or it's overcrowded, the temperature drops and the food releases water that can't evaporate fast enough. The food then stews in its own juices instead of frying. Wet vegetables and adding the sauce too early cause the same problem.