FYI Most of the time, my Zwift rides upload automatically to Strava, but very occasionally it fails to do so, requiring me to download from Zwift and upload manually to Strava.
Hi Claude, Usually this is caused by a network timeout while saving. If you want us to diagnose, the best thing to do is open a support ticket and include your log.txt files for your rides. We can walk you through how to find those in the ticket. The good news is when this happens you can always download your fit file and manually upload it as you’ve found :)
Thanks Crystal. Because this happens rarely, I’ll go with the network timeout theory and just manually upload to Strava if it happens again. 
Any chance Zwift can make accessing the .fit files easier, from within the app for the occasions where the uploads to fail?
Would be nice to have the option to manually upload, click a link that opens Strava and also has the .fit right there. Maybe a future update enhancement.
It seems odd that I can do a whole ride fine, with no network issues and then trying to save and upload it never makes it over to Strava.