The Purchase Order may have been already invoiced or even completed and so it not being available to match, could be correct. To check:
Log into Dimensions
On the Home tab, click on Creditors and then select Outstanding Purchase Orders
Search for the order using the field name Order No.
If the order does not appear, this means it is already complete. If it does appear, check whether it shows as invoiced - if it does, then it cannot be matched and so it not appearing is correct.
If you are still unable to see a reason for the order not showing, please raise a new case online and reference the title of this article. Please also include the order number or numbers affected.