Withholding tax calculation in supplier invoice

Summary

When I create a supplier invoice, withholding tax isn't calculated

Content

Hi,

I need an help with the calculation of withholding tax when I create a supplier invoice. 

When I create a supplier invoice transaction tax is calculated, but not WH tax. I've already created a supplier and I've inserted all the attributes related to my WH tax in the "transaction tax" options in Supplier profile page. 

Have you an idea to solve this problem? 

Thank you 

Federica