I bought a brand new Yamaha upright; as of today, it is just over a month old.
There is one key, a specific C# that sometimes, when I play with a mezzo-forte intensity or higher, the key sinks as if it had already been pressed, and the sound results in being pianissimo.
But the truly strange thing is that if I press the key individually and in every possible way, the key always responds perfectly...
The problem only occurs following a combination: right pedal + a few even random chords; after this combination, it frequently happens that if I press that C# with intensity, the key emits a very soft sound and the sensation on the finger is of a key whose escapement has not yet been reset.
Looking at the action from above, no strangeness or friction is noticeable. I thought it might be due to summer humidity, but the fact that it only happens on that specific key seems strange to me.
