Purchase order remains in open status once an invoice is matched

There are a number of purchase orders (raised to different suppliers) that are remaining in an open status once an invoice is matched.

The amount of the invoice is for the full order amount, so I would expect the order status to change to 'closed for invoicing' for a 2 way match.

The discrepancy does not appear related to timing or tolerances.

Please can you offer advice on the process for the system changes to the purchase order status, and why this does not appear to be working for some purchase orders that are fully invoiced.

Many thanks