I have a project for which I am attempting the following steps:

1) Extracting 5.1 DTS audio files from MKV files with MKV Cleaver.
2) Converting the audio to 6 channel WAV files using convertio for the purpose of editing in Audacity.
3) Edit the WAV file in Audacity.
4) Use Audacity to export the new 6 channel WAV files.
5) Reconverting the audio back to DTS on convertio.
6) Remuxing the new DTS file with the h.264 video stream from the original MKV using MKVtoolnix.

I've done all this successfully with a few shorter clips. But now I'm running into a problem.

For the third clip of the project, when I get to step four from above, It only seems to be exporting the first channel to the new WAV file.

The third clip is definitely the longest one, nearly 25 minutes, but I wouldn't think that would make any difference other than taking longer to export. And I'm pretty sure I'm using all the same settings I was using when the exports were successful, including making sure "Use custom mix" is selected under Edit -> Preferences -> Import/Export.

Any brainstorms as to a reason why Audacity would suddenly not be combining all the tracks into the new WAV file?

I'm happy to provide screen shots of the "Part 3" of the project, and/or any of the settings thereof, if that would be helpful in some way.

Thanks for any help!