A simple update doesn't update custom table

Hello Experts,

I'm trying to update a custom table ( zrerate_dtl ) field ( adj_amt ) after check some logic in the following code. However, field ( adj_amt ) doesn't get updated at all. Can anybody find any issue with the following code and suggest how to rectify it, please?

  clear: lv_tabix.
     loop at git_zrerate_dtl into gwa_zrerate_dtl.
       lv_tabix = sy-tabix.
       READ TABLE git_zrerate_dtl_old into gwa_zrerate_dtl_old
         WITH KEY zrerate = gwa_zrerate_dtl-zrerate
                  vbeln   = gwa_zrerate_dtl-vbeln
                  kdmat   = gwa_zrerate_dtl-kdmat
                  posnr   = gwa_zrerate_dtl-posnr.
          IF sy-subrc = 0 AND gwa_zrerate_dtl-adj_amt <> gwa_zrerate_dtl_old-adj_amt.
             gwa_zrerate_dtl-adj_amt = gwa_zrerate_dtl_old-adj_amt.
             modify git_zrerate_dtl from gwa_zrerate_dtl index lv_tabix"sy-tabix
             transporting adj_amt.

             update zrerate_dtl
             set adj_amt = lw_zrerate_dtl-adj_amt
             where vbeln   = lw_zrerate_dtl-vbeln
               and kdmat   = lw_zrerate_dtl-kdmat
               and posnr   = lw_zrerate_dtl-posnr.
               COMMIT WORK.
          ENDIF.
     endloop.

Many thanks in advance.