Note: this article also applies to the Korg M50.
You are working on a song in the internal sequencer, and you find that you cannot change the Track that you are recording on. It seems "stuck". Or, when trying to change certain parameters such as "Transpose", you are unable to do so.
1. A key, pad or damper pedal is being held down while you try to change tracks. The M3 will not allow you to change tracks (or change certain parameters such as Transpose) when notes are being sustained. This can sometimes be a "silent" stuck note that is sustaining, so even if you don't think any notes are being sustained, you should check this out. You can usually clear any stuck notes by changing Modes (i.e. Seq -> Combi then back). This can also occasionally happen when the M3 is hooked to a computer, and data is being echoed back into the M3. You might try temporarily disconnecting the MIDI cables.
2. The damper polarity is reversed. This can happen without you realizing it. Normally if the polarity gets reversed, it's obvious because the notes will sustain with the pedal in the up position and damp when you depress the pedal - but if the polarity is switched and you then change songs without touching the pedal, the sustain will stop, leading you to believe the polarity is correct when it's not. This is when you will be unable to change tracks. It's not until you press and release the pedal again that you discover the notes ringing with your foot off the pedal. (The fix is to simply reverse the polarity in the global mode/controllers page and then PRESS AND RELEASE the pedal.