This is covered I guess by David’s comments, but the problem may be with the reed gap — the distance between the reed plate and the open end of the reed. The gap may need to be opened slightly, by applying pressure (carefully) to the open end of the reed. You can use a business car, or a plastic toothpick, or even your finger to lift it slightly several times. But you should be able to see if this is the problem. If the gap is too narrow or too wide, that will be visible in comparison with the other reeds. If you search “gap” and “gapping” in the Forums here, you’ll find lots of comments about this. I adjust the gapping almost every time I tune a melodica.