There is too much water. Foam will expand when it is hot and shrink when it is cold, and there is not enough support. I used to steam cakes myself and found this problem. Just add the right amount of powder. I guess your dough is too soft. Although it is the same procedure, it is sequential. If you put it back for a long time, it will go too far and become soft. Or the flour is reduced.

1. Mix flour with cold water in summer and warm water in winter. Mix flour and dough in winter earlier than in summer 1 ~ 2 hours, and pay attention to adding water when mixing flour. 2. Knead the dough several times to make the starch and protein in the flour fully absorb water, so that the expressed gluten is sincere. Good dough should be stored at a certain temperature, 30℃ is appropriate. 3, when the noodles have risen, we must master the degree of fermentation. If the social group is already in a honeycomb shape and there are many small ones, it means that it has been fermented. The bigger the eyes of the honeycomb cube, the older the yeast is, even too much. 4. Steamed buns are cooked in cages, so they should be cooked. Winter is about 15 ~ 20 hours, and summer is shorter. When the steamed bread is put on the cage, the water in the pot must be turned on, and the atmosphere should be seen for 10 minutes. 5. The joint between the steamer and the pot mouth must not leak, and the leaking place must be blocked with a wet cloth. Cover the lid tightly when steaming with aluminum pot. 6. When steaming steamed bread, the pot must be heated with cold water and gradually raise the temperature, so that the steamed bread blank is evenly heated. Don't steam steamed bread with hot water or boiled water at first, because steamed bread is easy to pinch your hands. If the steamed bread is a little yellow, spray some white vinegar and it won't be yellow.