Most likely the Fee Arrangement for the Matter may have been set to non-billable when the Invoices were posted.  If there were actually dollars billed on the Invoices and the Invoices are not showing, that typically means no AR was created upon posting of the Invoices.  This occurs if the Fee Arrangement is set to Non-Billable.  You can test this by doing the following:

1.  Go to the Matter record, change the Fee Arrangement to Non-Billable, and save the changes.

2.  Go to Billing Manager and unpost the Invoices.  Do not void and unpost the Invoices.

3.  Go back to the Matter record, change the Fee Arrangement to Hourly, and save the changes.

4.  Go back into Billing Manager and post the Invoices.

After doing the steps, check Matter Status to make sure the Invoices are now showing.