sales tax not working

Dear All,

I have setup sales tax and linked to vendor group .

item sales tax group also created and attached to item.

but issue is while creating purchase order sales tax was not calculating if i attach sales tax group to particular vendor it is working fine.

should we give sales tax group to every individual vendor ?

if we give to vendor group it wont take for all vendors?

please give me some solutions.

Thanks

Regards,

Ashok