On ME23N screen follow the menu path Environment -> AC Commitment Documents. Do you see any FM documents? I believe your PO does not create any FM documents.That is why there is no budget consuption. The GL account needs to be explicitly specified in the PO.
When it gets to the SES point, assume you do not want to inherit FM account assignment from PO and you activated FLG_SES_OVERWRITE, you can explicitly enter something (GL account, etc) in SES that will work with FMDERIVE rules to derive FM account assignment.