Why did my QuickBooks sync stop working?

When you establish your QuickBooks Online sync in Full Slate, you tell QuickBooks that you're giving Full Slate permission to access your QuickBooks data. This permission expires after six months, requiring you to grant permission again.

To do this, go to Setup > QuickBooks Sync, disconnect your connection, and then re-connect. In the process, you'll grant Full Slate permission to access your QuickBooks data for another six months.