Hi NG,
There can be many factors for no plant in line item 2.
There are standard SAP plant determinations as described by Gurtej. You can check them.
ale order is getting created from IDOC as a result from PO.
Kindly check the IDOC segments for Plant value, what i doubt is that you will get plant value for line item 1 and not for line item 2 in IDOC.
You can check and confirm back.
Thanks
Kartik