Regarding Withholding Tax data updation

I have posted a AB document using FB1K.Whenever i go to FB1K transaction & put the vendor and click on Process open items,i am getting 1 pop-up saying "Withholding tax ignored". Then i will post the open items. This will create a AB document type.

Then if i go to FB03 for this document,there will be zero amount updated for both the line items. This is happening eventhough the vendor is liable for Withholding tax.

But if i post any other document like KR,RE,KG,its properly updating the withholding tax.

Can someone help me how AB document is getting updated with zero amount.

Thanks,

Aparna.