As I suspected,
Routine 923 for access sequence might control the appearance of the access in the access sequence.
I went through SAP Note 178824 - Withholding tax from SD for countries other than Argentina and Std Routine 83(Include LV61A083) for access sequence. Where I can imply that the issue is with your custom routine 923 used in requirement.
If you observe, that Note & Std Routine 83 for access sequence only mentions of Country & Tax rel.
Whereas, field like Sales Doc Type, Tax classification for Material and Item Category are not compatible to them or considered for processing.
Thus, I would suggest, to make that table enable for processing, take help of ABAPer in your team and make necessary changes in your custom routines and related changes like FM or report.
Hope that helps you.
Thanks, JP