Value field not updated in COPA Document.

Hi SAP Experts,

After the billing doc when accounting docs in generated it is having 4 line itmes 3 are for different types of taxes. Now along with Accouting doc there is also a COPA doc generated. My issue is that out of the 3 different types of taxes that we see in the line item of acc doc (FB03), one tax is missing in the COPA doc.

I mean the Value Fields only updates 2 taxes and the 1 is been skipped.

I have 2 question:

1. How does this value fields gets picked up in COPA docs?

2. How to check why the 1 tax line is been skipped ?

Thanks in advance for your reply

Regards.