[Alan Brinton]

It doesn’t look like the key spacing is far enough off to create a problem. However, often it’s easy to adjust and even out the spaces just by inserting a razor blade or other thin blade between keys and jiggering the keys one way or the other. The B could be worked a bit to the right, and there’s room to fiddle with the C key to see if that makes a difference. There has to be some reason for the problem you’re having. Something with the reed seems most likely, especially if you’ve worked on that key, gapping/tuning, but the entire mechanism of that key is possibly relevant, including the spring tension, which could affect how far the key travels up and down. But that would be noticeable in the key action, and it seems unlikely on a new melodica. Sorry if I’m not being much help here!