I noticed you've taken some troubleshooting measures to resolve the issue, JMFS. Thank you for your efforts, as they assist in pinpointing the root cause.
Aside from that, did you somehow encounter specific error messages? Details about it will help us move forward to more specific troubleshooting steps in fixing the problem. Moreover, you can follow the provide troubleshooting steps in this link: Spreadsheet Sync troubleshooting.
While we don't have a definite reason of what's causing the problem and a resolution, it is best that we contact our Customer Support Team. They can pull up your account, review your data, and provide additional troubleshooting steps. You can reach them during their support hours: Get help with QuickBooks products and services.
Keep us posted on this so we can also note the best ways of fixing the issue. We'll be around if you need anything else.