Availability check in sales order not available after delivery is created

[MariaPaula]

Hello Community, 

I have the following situation:

Material	Stock
A	0
B	5
C	10
D	0

We create a sales order with material A requesting 30 units but material A doesn't have stock. As it has material B and C as substitutes, sales order gets created like this: 

Item	Material	Order Quantity	Confirmed Qty
10	A	30	15
11	B	5	5
12	C	10	10
13	A	15	0

Then customer is creating the delivery with lines 11 and 12 as they have confirmed quantities. Units of line 13 goes to backorder. Then customer received some stock for material D, they want to execute availability check so that sales order gets updated with a new line with the available material and quantities for backorder are reduced. 

However, once we create the delivery we are not able to execute item availability again. Line with the original requested material has schedule line category CN and the substitutes have schedule line category CP.

Could someone help us mentioning why availability check is not working after we create the delivery if the sales order allows partial deliveries and how can we make it work?

Thanks a lot.