(a) Only finalised invoices are synced so please check that the invoice in questions has been finalised.
(b) Invoices with issue dates before the sync start date will also not be synced. Please amend the sync start date to an earlier date which covers the invoice in question and issue a different sync request.
(c) MechanicDesk may have attempted to sync the invoice and failed. This could be due to many reasons. Please check the sync status box and make sure that there were no errors thrown for the invoice.