The change date of PR in ECC does not update in SRM

Hi,my experts:

I work in SRM7.0, PDP Scenairo

I create PR in ECC ,and then the PR will transfer to sourcing in SRM .But if I change the quantity of the PR ,the update will not happen on the SC(the PO has not been generated in sourcing ).

Is it a system error ?Or others ?

BR!

Alex!