VBEP-WMENG field is not getting updated

Dear All,

    I am trying to change the order quantity(vbep-wmeng)  in VA12 through an userexit based on the quantity entered in additional tab b.

In form   USEREXIT_MOVE_FIELD_TO_VBEP I have added the code as.    

    xvbep-wmeng = xvbap-kwmeng.
     xvbep-updkz = 'U'.
     modify xvbep index sy-tabix transporting wmeng updkz.

But the vbep table is not getting updated. This userexit is also not getting triggered.

Kindly suggest a solution.

Thanks & Regards,

Neela M B