Sometimes you won't be able to trigger an instrument on an Audio Instrument or MIDI track as expected. This can happen if you inadvertently disable the Record-Enable button on the selected track. (Normally, a track's Record-Enable button will be enabled when you select the track.)
In order to monitor Audio Unit instruments, MIDI instruments, or incoming signals, this button must be turned on. When turned on, the button will be red.
When the button is turned off (it appears gray) on a selected Audio Instrument track, then incoming MIDI from your controller will not be able to trigger the instrument. You will, however, be able to hear anything that's already recorded on the track during playback.
To resolve the issue, simply reactivate the button as shown above.