FI substitutions for custom fields in BKPF table

Hi,

We are having trouble making the substitution work for the custom header fields in table BKPF.

We have tried straight substitutions, exits and populating the fields on the interface table ACCHD.

Nothing works. But the substitution for the custom fields in BSEG works.

We did activated these substitution rules.

Do we need to have blank instead of "X" in the GB01 table for the field BEXCLUDE ?

Did we miss any thing here or we cannot use substitutions for header custom fields ?

Thanks

Steve