Tax amount (VBRP-MWSBP) shows 0 on billing document

Hi,

Tax amount (VBRP-MWSBP) shows 0 on billing document, while if we drill down to the conditions, the tax type and amount is shown.

This Billing document is Z type and the pricing procedure in use is also Z.\

What controls the display of tax amount on Billing document (VBRP- MWSBP)??

Thanks,

SS