Stero recording suddenly stopped working

OSX 10.13.4 High Sierra
Audacity 2.2.2
Zoom H4n

I’ve been doing an overdub multitrack recording a 4-part recorder ensemble piece all day using a Zoom H4n as the mic, with the line-out port from the Zoom going into the Mic-in port on my Mac. Each part has 2 tracks to help in overdubbing patches and re-takes, for a total of 8 tracks plus a Click track when needed. It was recording in stereo for hours and everything was going just fine, and then suddenly the next little bit I recorded was only in mono on the left side. Nothing was changed at all, it began doing this behavior on its own. The Zoom is set to stereo and both channels are responding. No XLR mics plugged in, Mono Mix on the Zoom is off.

Why would this start happening after hours of no problems? Is there some sort of time limit and the software gets fatigued? The piece itself isn’t even 5 minutes long so far; I’m about half-way through the basic recording of each part before I start any post-recording fixes.

I know about “split stereo to mono”, but there are lots of separate regions on each track, so I’m not sure what will happen.
Thanks for any help or insight.

Fixed Typo: Stereo recording suddenly stopped working


Perhaps a problem with the Zoom. Perhaps a glitch in the USB connection.
If you reboot your computer, can you reproduce the problem?

Thanks for the suggestions- a reboot of the machine seems to do the trick.