The most common reason for an order not showing for matching is that the order has been set to Completed in Access Dimensions.
In order to be matched to an invoice, the order itself must be outstanding in the Dimensions purchase order batch. You cannot match an invoice to a completed order.
To check this in Dimensions:
Navigate to the supplier record and select Outstanding Orders (note that this may also show as O/s Ords)
Check the filters and remove the tick from Outstanding and tick Completed
If the order number you are expecting to match to appears in this list, then the order has been completed and you will not be able to match the order to an invoice.
Closing an order is an action that can be taken against batched orders in Dimensions to indicate that the order is no longer required. Note that closing an order in the batch is a permanent action - this cannot be reversed.
If the order was closed in error then you will need to arrange for a new order to be entered into the system so that the invoice can be matched.