Why can I delete some purchase orders but not all?

Users may see that some purchase orders can be deleted while others can't.

The Flowlens application allows purchase orders to be deleted right up until such times as they are given a status of 'Received'.

To receive a purchase order, select 'Receive items' from the 'Actions' drop-down. This option will only display once a purchase order has been marked as 'Sent'.