@MisterBill - here's things I can think of:
1. The invoice as the "Waiting For Sync Approval" checkbox checked on.
2. You haven't clicked Save after adding the line items (if entering the invoice through the regular interface).
3. You haven't called the "Send to Desktop" action (if you are entereing the invoice using actions instead of the regular interface).
4. Your sync isn't on - in general - so nothing is syncing!
5. The invoice is for a customer who hasn't successfully synced to QuickBooks yet (should be listed under QuickBooks > Resolve Conflicts).
6. The invoice line items don't add up to a positive amount (but this should be listed under QuickBooks > Resolve Conflicts).
7. It is being rejected by QuickBooks for whatever reason, and is listed under QuickBooks > Resolve Conflicts.