There may be several reasons why the skin of yakisoba is hard when it is steamed:

1. Excessive kneading of the dough: The dough will stimulate gluten during kneading, and too much gluten will make the skin hard. Therefore, when making the dough, avoid over-kneading the dough and simply mix the flour, water, oil and other ingredients well.

2. Overly dry crust: If the crust is exposed to the air for too long when it is being made, or if it is not covered properly during storage, it will become dry and hard.

3. Steaming for too long: If the siu mai is steamed for too long or the fire is too high during the steaming process, it will make the skin become hard.

4. Insufficient water: If the dough is made with insufficient water, or if enough water is not added during the rolling process, it will cause the skin to become dry and hard.

In order to solve the problem of hard skin of yakisoba, you can pay attention to the following points:

1. Control the time and strength of kneading to avoid over stimulating the gluten.

2. Let the dough rise for a while before rolling it out to increase its softness.

3. When rolling out the skin, take care to roll it into a thin and even skin to ensure that the steamed skin has a fluffy texture.

4. When steaming siu mai, control the time and fire to avoid over-steaming.

5. During the process of making the dough and rolling out the skin, add enough water to ensure the softness of the skin.