Garageband 10.4.2 on Big Sur Macbook Air. Can

I've been using GB successfully for a couple of weeks. I'm a bass player. I connect via a UMC204HD interface. I drag an MP3 into New Project, record the bass on a new track then send it to the guitarist (that track only) for him to mix. Today I went to add the bass, but although I can play along to the track when not recording (to practice my part), and I can see that there is a signal being received for that track, when I press record the track turn s red but just flatlines, no signal? Preferences in GB and on the Macbook air are set for the interface, input and output, and the track is recording enabled and not muted. I've been trying to sove this for four hours now! Any ideas?