That’s the odd thing; I wasn’t getting any errors when running the workflow as far as I could tell. I’m new to this, so it’s possible there was something in a log somewhere that I’m just to daft to have seen. 
The symptoms I did see is that:
- When testing the workflow I’d see the little green checkboxes in the nodes in the main workflow, but never any in the sub workflow.
- No files would ever be created in the GitHub repo.
The only error I did see was some “node is out of date” errors. I tried testing the workflow “as is” with the out of date nodes, then re-tried after following the instructions to remove the out of date nodes and re-add new ones. Still no luck.
Not a big deal though. I found this workflow from user @ihortom that works perfectly. I extended it a bit to add Slack notifications similar to the workflow I linked to in my question above. So I’m all good and happy now. 
Thanks for your quick reply!