I have a baby grand piano and the middle D key has been getting seemingly 'clogged/stiff', when I press it once after first sitting down it usually works fine but then will become stiff and not sound, feeling as if there is a piece of felt blocking the key.
Upon taking the front lid off, I noticed that the jack is occasionally falling further than normal after releasing the key. I also notice that the jack is a bit further to the right than other keys, if that means anything. Sometimes lifting the key up a bit will make the jack go to where it should be but sometimes it won't.
The best solution I have come up with so far is putting something between the key and the jack, which stops it from falling too far but the problem still occurs when I press the key repeatedly quickly without fully releasing it. If I repeatedly press it slowly, there is less stiffness at first but the sound becomes weaker gradually to the point where there is no contact between the hammer and strings.
My current research suggests issues relating to the guide and balance pins but they don't seem to be the problem, the key appears to be better aligned than most of the others.
What is causing this and how do I fix it?