Out of Tune Octaves

Are some of your octaves slightly out of tune?... specifically on some holes of some keys of harmonica... even when they're new... out of the box or customized?

If so, try to find one right now and reproduce it... the newer the harp the better.

Play that octave again (e.g., 1 draw/4 draw octave... we're of course talking about playing them both at the same time). If it's out of tune, try OPENING YOUR NOSE while you play the octave. Is it in tune when opening your nose? If so, then your tongue position was slightly bending one of the reeds, most likely the lower reed. This is also why it happens on some harps and not others... your normal location of rest for your tongue in your mouth while playing octaves will only affect some pitches. When opening your nose, the pitch of your oral cavity drops extremely low, due to the fact that the nasal cavity is also part of the resonant chamber... tuning your mouth to a very low pitch, a pitch that doesn't affect any of the reeds (slightly bending one in this case).

Opening your nose is just a quick check. If this is an issue, the long-term fix is to lower the back of your tongue, making the largest tone chamber you can, imagining that you have a tennis ball in the back of your mouth. This will again tune your mouth to a pitch too low to affect the tuning of any of the reeds you're playing in the octave.

P.S., most players are too quick to blame the harmonica, BUT, sometimes the harp IS out of tune, but this is a way to check ;-)