Some keys on the piano are obviously too small. What if you want to adjust them yourself?

First, check whether there is a foreign object stuck in the key. If not, take off the covers on the top and front of the piano, and then you will see all the hammers that strike the strings. Looking down the hammer, there is a wooden bar similar to a small shovel, and none of the keys are connected to the hammer. Then, you can see which key doesn't ring and whose stick is not good. Adjust yourself. If it's boring, see if the cloth of the mute pedal is inflexible and blocks the hammer.