How to track revenue with content groupings in GA?

Hello here.

I have setup GA to send content grouping information via this code:

gtag('config', '[my-account-number]', {'content_group1': '[my-content-group]'});

But it looks like revenue sent when a customer purchases something gets not associated to the content group it belongs to. So, I am completely unable to understand what group is giving me more revenue. When I analyze my data, all data that include a transaction has "(not set)" as content group. Any idea? How to solve that?

Thank you in advance for any help!

Fab.