I think I found a solution. let me know if it works for you

When you go back to a saved 'item receipt' and click 'Select PO' from the top taskbar. You'll see a check next to the PO you received the items for. Click on that line to deselect it then select it again. Click 'OK'. You should now have the option to receive more inventory towards this same PO. Click 'recalculate' and then 'Save'

I hope that helps.

Abe