Issue related to Sales tax

Hi

I am facing problem while doing PO, If I enter qty, price it is calculating all the values like ED, cess, SHcess etc. properly.

But I when I go for changing qty or price when I am in the same PO , It should calculate all the values accordingly, but calculates all the values except CST(It is taking the same value bydefault from old PO, and If I see condition base value in condition details available in PO screen, it is showing condition base value of old PO bsaed on this amount it is giving the same CST for every PO). Why it is so? what is cindition base value and where is the setting for this?Explain in detail please. And I am working on 4.6C version.

Regards

venkat