Sorting in Table Control

Hello All,

I have table control & I added sort button with code ..

in top program ..

data:  fld_list   LIKE  TBL_CTRL-cols,

wa_fld_list LIKE LINE OF fld_list.

in PAI..

  when 'SORTU'.

       READ TABLE TBL_CTRL-cols INTO wa_fld_list WITH KEY selected = 'X'.

       IF sy-subrc = 0.

       SORT  IT_ZPRODUCT_T STABLE BY (wa_fld_list-SCREEN-name+14) ASCENDING. "

          wa_fld_list-selected = ' '.

         MODIFY TBL_CTRL-cols FROM wa_fld_list INDEX sy-tabix.

        ENDIF.

I am facing this problem :

when i select any column and click on sort  button nothing happen..... why ?! 

regards ,

zakyaih