Dear Yusuf,
Recheck the wht configuration steps under extended withholding tax:
1. Withholding tax country:
2.Define Withholding tax type for Payment Posting
3.Define Withholding tax code
4.Assign withholding tax types to Company Code
5.Assign Accounts for Withholding Tax to be paid over
6. Assignment to Recipient types.
also you may link: http://scn.sap.com/docs/DOC-48399
Regards,
Charmaine