< The symptom is that the reed will respond very poorly with low
< air pressure - not completely silent but pretty strangled/sickly.
< Blowing harder will usually cause it to 'pop up' to a more normal
< sounding note >
This sounds like a valve problem to me. The wind-saving valve is reluctant
to close off the chamber unless you blow (or draw) quite hard. Then it
closes suddenly, hence the pop. Don't fiddle with the gapping until you've
checked this first. A good test is to see whether warming up the harmonica
improves things. If it does, it's not the gapping but the valves. It could
be that the harp needs warming up or that the valves need cleaning or
replacing. G is the man to advise. Assure yourself that it isn't the
valves sticking before you fiddle around with the gapping. If the gapping
was OK once it's hardly going to go very much astray, whereas valves, the
annoying little b**ggers, are supremely temperamental.