Q: Some notes on my harmonica seem like they’re going flat. What causes this?
A: The most common cause for hamonica notes going flat is residue or moisture build-up on the reeds. This is why you should have a clean, dry mouth when playing harmonica, and why you should tap out the harmonica against your hand and wipe it dry after playing.
In rare cases, playing too loud can strain or fracture the reeds; this will result in a drastically flat note or notes. A harmonica with strained or fractured reeds should be replaced or repaired.