I’m assuming it’s a PO for a stock transfer if you’re creating deliveries.
What are the settings in the delivery tab for delivery complete, final delivery, part delivery/item?
It could be that the items were already shipped on another delivery (check the PO history) or it could also be that the customer created for the plant doesn’t allow partial deliveries.
To check, go to the shipping tab of the PO to find the customer number, then display the customer with tcode XD03. Enter the company code and sales org so you can display the Sales area data. When you’re in the customer record click on the Shipping tab of the sales area data and see what the setting is for Partial Deliveries per item.