This is standard functionality of SAP. When carrying out partial GR against an inbound delivery, the HU for which GR has already been posted will not be displayed until after the full quantities have been received.
When a HU is posted GR, it gets de-assigned from the inbound delivery. When the delivery Total Goods Movement status is C, the HUs would be read via the document flow (VBFA table). Otherwise, they are retrieved via their assignment. Therefore, if Total Goods Movement status is B (partial), only the HUs that have not yet been posted are displayed in the packing screen.
Note that the HUs that are not displayed in the delivery packing screen can still be displayed using transaction HUMO.
In order to see all the HUs in the Packing Screen of the inbound delivery, complete the full goods receipt.