I record some MIDI data but the synth does not sound

I have created an external instrument for my Novation Summit synth. I create a track. I can control the synth using another controller. So the MIDI input is set to the Controller and MIDI output is set to the Synth and sounds are made when I play the controller. This tells me data MIDI is flowing from the Controller through Cubase 11 to the Summit and audio is coming back from the Summit into Cubase. Here’s the weird part. I record some MIDI data (and the synth plays during recording), but the synth does not sound when I play back the track! I am completely stumped.