Quickbooks Online Sync Issues

We've been using the QBO sync for the past month or so.  It generally does what the documentation says.  However, I'm having a problem where a sprinkling of client invoices aren't being synced to QBO.  A subset of these invoices--I found out through doing a few QBO searches--had an invoice number that matched a vendor bill that was previously in QBO from a couple of years ago (and thus never existed in XTRF).  By preemptively re-numbering my vendor's old invoices in QBO I could avoid the problem ahead of time.  But now I'm still having invoices that don't sync and as far as I can tell the invoice number either doesn't exist at all in QBO or is so old that it's not showing up in a standard search.  

If anybody has an idea of where I can go from here, I'd love to hear it. Or if you're having the same issue, let me know!