my A harp plays fine except on 3 blow hole, when trying to get a clear single note blowing it is very fuzzy sounding almost like a kazoo. every other note is fine including the 3 draw. i have tried some other different key reeds in the same harp and they are fine, and tried playing the dodgy A reed in another harp and it still plays the same fuzzy way. so it is obviously the A reed. would you recommend a new reed or is there a repair i can do to the reed.
Shim = feeler gauge around 0.05 or 0.10. If you run the feeler gauge down the length of the reed in the slot it could remove any very small burs on the slot or reed. If you notice it is very tight on one side it could be that the reed is not centred in the slot as Boris mentioned and you can use the shim to ease the reed over carefully. You can see if the reed is not centred properly by holding the reed plate up to the light and checking the gap on each side of the reed.
If you are unlucky it could be that the reed is on the way out and there are microscopic cracks in the reed which is affecting its ability to vibrate properly hence the buzz and ultimately it won't play at all.