Withholding Tax is not calculated after validate invoice

Hi

We have a big problem.

The withholding tax is not calculated after validate invoice, i have done all setup in payables options and i have created a withholding group and activate the option at site level for supplier.

Someone could tell me a checklist or step by step for do this ?

I have done this:

1.- activate the option "use withholding tax" on payables options

2.- automatic offset: NONE

3.- create a withholding group

4.- create tax code

5.- create supplier like tax authority

6.- allow withholding tax at supplier level and site lvel

after this, i have created an invoice but this not working