how to reset the Calculate_tax_item function module

Hi expert,

I am using calculate_tax_item function module for calculating tax for purchase order in item wise. If the purchase order having only one item means its showing result correctly...but if the purchase order having two item means it takes 1st item also after i refreshed that internal table... i don't know how to reset that function module but option is there to reset...its not working after i gave reset = 'X'. Guide me to solve this issue...

Regards,

Rathish