"In the table manager(fields) for the invoice, it shows the calc as unstored."
This is normal for any calculation that refers to a field in another table.
I can imagine any number of reasons why the calculation "doesn't work". If you can spell out the relationship linking your two tables and post an example of your calculation maybe one of us can take a shot at helping you out.
Ah, actually its been resolved. Short answer is it was a typo. Thanks!