The most common cause is that the 'Sync items from' date in the Accounting integration set up.
In the integration you can set a date from which to sync from, and any invoices created on an earlier date will not post across from Buildxact to your Accounting package
The 'Received' purchase orders here in Buildxact were created on 19th September 2017
But in the accounting sync settings window the ''Sync items from'' date is set to the 21st September, which means those above "Received" invoices will not post across to the Accounting package.
To amend this, go into the Accounts integration settings and change the "Sync items from" date to an earlier time and try syncing again.