Have you accidentaly placed one of the other midi tracks in solo mode? I went through something similar and wondering if that could explain it.

I could use the OT or an external keyboard to trigger an external synth (in this case, A4 and iPad) when I was on the active midi track that I wanted to use. I’d then be able to record the midi pattern as I saw fit but once playing back no sounds were played. I was going bonkers trying to sus it out until I realised I had solo’d another of the OT tracks.