There is a known issue in the ConnectWise API where sometimes a few expenses that are part of an expense batch won't sync over to QuickBooks Online. We are working with the ConnectWise team for the issue in the API to be fixed.
1) Navigate to the missing expense(s) in ConnectWise. Select More>>Unbatch
2) Navigate to Mobius and sync the expense. The expense should now appear in QuickBooks Online.