Probably the C# tone hole is not closing. Use the leak light. Some horns have adjustment screws. On horns without them, you (meaning your repairman) will probably need to install or replace a bit of cork. Or bend a connecting lever. Or something. It's tricky.I've bent an Abbott Premier alto back into playable shape but the upper stack c is very stuffy: half the volume of the adjacent notes, muted, and it 'chatters' or wobbles at the start of the note. It's the only note misbehaving. Things look alright, but I'm a self taught repairman with three months experience so I don't know what else to look for except the next open pad. Is it open enough? Is the pad good, torn? Yes, it seems open enough and the pad looks good. What else could cause one dud note? I guess I'll just go over it in close detail righting wrongs as I go. There's a small dent in the neck. The neck/ body joint could be better. The mouthpiece might be mismatched. I haven't used a leaklight yet because all the notes except c are playing with a light touch. I'll just keep at it.