my sostenuto pedal is not working properly... When I press it down, the felt gets down so the hammers doesn't hit the string directly but when I release it the felt doesn't rise back completely so the hammers, mainly the lowers, gets stucked with the felt and some keys doesn't sound. I've tried to adjust the entire felt to the sides of the piano but it keeps muting and getting stuck my hammers... how could I fix it? Is it a regulation issue of the pedal system or could it be something else? Thanks
That's not a sostenuto pedal. It's a practice pedal.
Sostenuto works in a way that it keeps the dampers off the strings that are sounding at that moment. Like a sustain pedal, but only for specific strings.
What you have is a pedal which is attached to a lever which brings a curtain of felt between strings and hammers. It effectively dampens the sound when played, and works on all strings together. It's lockable - a sostenuto pedal isn't.
It is a simple mechanism, working on levers, so maybe a joint has seized up, and needs a little oil to lubricate it. You should be able to see how it all moves (or doesn't!) with the front of the piano off, revealing the strings, hammers and mechanism.