Probelm is excise rate in PO

Hi,

I have 4 raw material for which the Chapter id is common as 8538.00, for all the 4 item J1ID is maintain and also the cenvat determination the excise rate is set as 12 % from 17.03.2012 before that it was 10% but for only one item in PO it is coming as 12% in the tax code for remaining it is shwoing as 10% only for the same tax code, what can be the problem , chapter ID is same for all the material.

regards,

zafar