Open up your melodica and inspect the critical reed here.
It could be encrusted with corrosion or some saliva bits, in which case, carefully clean it.
It could be not quite centered in the slot. Use feeler gauges (buy a set they’re cheap and useful) to check the positioning on either side – it should be square in the center of the slot beneath. You probably can bent it just a touch to get it centered if its not. But that may be difficult to accomplish – exactly the right amount of force in the right place so as not to harm anything else of the sensitive reed.
It could be bent into or away from the slot slightly. This is a classic sign of a cracked reed in my experience. carefully flex the metal near the base so that it lays like the other reeds, just above the slot. This too is tricky, but less tricky than centering a reed.
If you have a pianica, I know you can buy replacement reed plates from yamaha parts.
If you have a melodion, I’m not sure about suzuki parts – but I know that I have a similar problem with my 37 key, hard blowing causes the b not to sound properly after its warmed up, and down by c/c#/d/d# the reeds clog up with moisture. very frustrating. firstname.lastname@example.org if you care to send some close-ups of what you find.
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.