This can happen if the guest's audio is never able to reach the cloud drive.
- Network Connection Problems.
- Firewall blocking dropbox. This happens a lot at corporate office spaces.
- Script or Ad blockers. Some anti-tracking script blockers will block the dropbox connection.
Once the source of the problem is resolved you can have your guest return to the recording page and click on the track to retry the upload from their local backup. Usually just waiting for the internet to be stable again will work.
If the upload is still failing, you can then have them right-click on the track and select 'Recover from backup'. This will export the local recording to their filesystem. They will need to send you the file in this scenario. Sending them a dropbox share folder works well for this.