If it is just one note and you are pretty sure it is stuck you can
take the top cover off and see if there is any crud or lint blocking it. Sometimes you can slide a thin piece of paper under the reed and see if it's physically stuck...a gentle touch though.
If it's several blow notes often it is from blowing to hard, so if it plays when you use a nice light exhale...then you'll have better idea if it is you or the harp.
If you are a tongue blocker, it may just be gummed up, rinse in water.
Take a look in the back to see if something is lodged in there affecting the reeds.
Take off the cover plates & have a better look at the reed plates.
Remove the reed plates, carefully noting which way they sit, & see if anything has got inside.
harmonician wrote: I have bought four marine bands, and the first one I got, which is in 'D', and was OK in the begining won't play properly, it's tough to blow, feels blocked. What can I do?
I hate to keep ragging on the Marine Band but this is why I don't like them...and I'm assuming you mean the model1896 as opposed to the Special 20 or Marine Band Deluxe
With all due respect to the previous replies, removing the cover and/or reed plates is a chore and you don't want to rinse a Marine Band in water. The dang thing is nailed together (the Deluxe and SP20 are bolted). If it was a bolted harp, it would dismantle in a minute. If it was plastic you could run it under water or put it in an ultrasonic cleaner. The 1896 MBs are not maintenance friendly. Oh, you CAN work on them but a simple 2 minute job on Suzuki Bluesmaster could be a frustrating 30 minute job on a Marine Band.
The question I have is this: Is the ENTIRE harp hard to blow or is this limited to a specific hole are range of holes? Is it just on the blow or the blow and draw? With luck, if it's limited, you can just plink or move a reed or two without taking things apart. If the whole thing needs to come apart, you'll end up with 20+ nails rolling around.