Hi Blair-
The first thing to check is your backup logs. Navigate to Total Upkeep > Tools > Logs and check your most recent log file. If you can, paste that log into the forum here so we can take a look.
The most common cause of this is either a timeout or a memory issue, which can sometimes be fixed by changing the backup zip compressor to System Zip. Here’s instructions to change the compressor.
To summarize, navigate to Total Upkeep > Settings > Backup Process and change it to System Zip. If you’re still having trouble with System Zip, try reducing the compression level from 6 (default) to a lower number.