GarageBand stops recording my MIDI drum track

Just got my first MacBook Pro and I'm trying to use GarageBand 11 to record a song.

I have recorded a keyboard track via MIDI without problem.

I'm now trying to record the drum track. Each time I do, the drum signal is dropped part way through the recording. It doesn't happen after a consistent number of measures but I never get more than 2 or 3 minutes of drum track and sometimes only a few seconds. When it happens the GarageBand timeline keeps going, but it records silence for the remainder of the track.

What am I doing wrong?