collect statement does not work properly in smartforms

Hi,
i used collect statement in a colum in table in smartforms under main undow.....my code is

select  vbeln  zprs_role from vbrp into corresponding fields of table it_role
  where vbeln = wa_vbrp-vbeln.

  loop at it_role into wa_role.
   collect wa_role into itab.
    endloop.
loop at itab into wa.
      role = wa-zprs_role.
    append wa to itab .
    endloop.
i also have loop in table it_tab to wa_tab.

finaly i display role in text in smartforms but it pick the last value among the 4 value and diaplay it 5 times..
but in dibugging i can see that itab have 4 value after using collect statement
the structure of itab,it_role ,it_tab is

types: begin of ty_tab,
      fkdat type vbrk-fkdat,
      knumv type vbrk-knumv,
      kawrt type konv-kawrt,
      ps_psp_pnr type vbrp-ps_psp_pnr,
      post1 type prps-post1,
      arktx type vbrp-arktx,
      posnr type vbrp-posnr,
      fkimg type vbrp-fkimg,
      netwr type vbrp-netwr,
      vbeln type vbrp-vbeln,
      zprs_role type vbrp-zprs_role,
      end of ty_tab.