Did the suggestion by Aldevinas work for you? If yes, then may I close this ticket? If it's still a problem, then maybe you could write a Wish and let us know the Wish title text, so I can follow up.
More explanation @Paul:
When the issue occurs, should see if restarting presence or lookup helps. If so - that is the issue and should be sorted. First need to ensure that that is the issue...
Hello!
You can mark it as closed as option. If document is connected with other documents you can not delete it so you will not be able to delete PO, as you will not be able to remove Goods receipt...