I didn't really panic, but I was getting worried.
The draw A in the left-hand G-row of my recently acquired Dallas-Crabb Anglo was a bit flat. When I got it, a few months ago, everything had been in tune, if I remember rightly. But in the last week, the offending reed was getting flatter and flatter, and also sounded muted and didn't achieve even its flat pitch under light pressure, only when forced.
I took the end off to see if there was any debris in the chamber, but there wasn't. The set of the reed looked much the same as its neighbours, and the valve was equally inconspicuous. The reed was now seriously flat - 30 cents, according to my tuner.
So, still suppressing the panic, I decided to make my first attempt at tuning a reed. I removed the reed-pan, and got the reed out quite easily. When I had it in my hand, I noticed that there was some kind of dark brown deposit on the shoe, especially where the reed is clamped to it. The deposit was inside the slot as well.
"Cleanliness is next to godliness," as they say, so before doing any filing I got terpentine, a soft cloth and a toothpick and cleaned the brown stuff off and out of the crevices.
To check the flatness, I reassembled the end, and lo and behold! the offending note was as loud and clear as all the others, spoke immediately, and was in unison with the draw A in the C row!
To quote another adage, "A job begun is a job half done." In this case, a job begun was a job completed.
I then carefully played each note, and, listening carefully, noticed that a couple of other reeds were just starting to show a trace of this muting and reluctance to start quickly and to reach full pitch right away. so I cleaned them as well, and they came up bright and shiny, acoustically as well as visually. So I plan to clean all the reeds, a few at a time, with the prospect of playing a really brilliant concertina.
So the moral of the story is, even if a reed sounds disastrously off, it may just need cleaning, so don't panic!
I've learnt a lot about maintenance in this forum, but I don't recall reading anything about crud causing serious tuning problems. But it does!
I'll just have to postpone my first attempt at tuning - thank Heavens!