Applies to Launchkey MK3
Go to the Preferences (Options > Preferences on Windows, Ableton Live > Preferences on macOS) and go to the Link/Tempo/MIDI tab:
Ensure to select "Launchkey MK3" for the Control Surface:
and for the Input & Output select the second port listed:
Note that on Windows systems the DAW port will appear as "LKMK3 MIDI (Port 2)".
Your settings should match those shown here:
You'll know that the Launchkey is working with Live correctly when you see Session + Device/Volume/Pan/Sends pads light up when you hold down 'Shift'.
Incorrect settings may cause issues like certain notes not sending MIDI.