Have Gross/Net weights auto populate

It is an array formula, not a code, so whenever you make change to the formulas, you need to do:
CRTL+ shift+enter( Hit these buttons at the end of the formula)

NOT just Enter.

If you do enter only, you will get a funny result.
I have also assumed you want to use both columns(Item no and quantity) to compare(match). If it is just an item only, I need to adjust the formula(It is actually becomes simple one)