Update: I've solved this. Asked my wife to plug her electric guitar in instead of my acoustic ukulele, and the signal was much stronger. Turns out that as the batteries drain in the piezo pickup in my ukulele, its output signal gets weaker and weaker. I thought it would stay the same and then just stop working when they went flat. I put in a fresh set of batteries and the signal level came up to be pretty much the same as the electric guitar. Leaving this here for anyone else who encounters the issue and doesn't think of the obvious answer!
Trying out recording the amp for the first time. Everything works, except the record level into Garageband is extremely low. Even with the amp's Drive knob at 100% and my instrument's output volume at maximum, the waveform barely registers in Garageband.
It sounds plenty loud from the amp. I've tried several of the presets and they all suffer from the same problem.
I looked at the Sound Preferences pane for the V20 but it has no input level control, so it's not that, and in GarageBand, the record level slider is also greyed out.
What am I missing?