Hi,
This means that you probably have a view override on the invoice from an old version of HikaShop which didn't support the display of custom item fields on the invoice.
If you're talking about the invoice you can print in the backend, then you want to check the file order / invoice.php
For the invoice available on the frontend it's the file order / show.php
You can check if they have an override in the menu Display>Views. There, edit them for the template you're using and check the version number at the top of the file. It should be 4.2.3 if you don't have an override.