double click problem in module pool

Hi Experts,

iam displaying itab_vbak containing quotation number and details of quotation in table control

of module pool programing.and I WRITTEN THE CODE AS FOLLOWS

case sy-ucomm.

WHEN 'PICK'.

GET CURSOR FIELD WS_FIELD LINE WS_LINE.

IF WS_FIELD EQ 'ITAB_VBAK-VBELN'.

READ TABLE ITAB_VBAK INDEX WS_LINE.

SET PARAMETER ID 'AUN' FIELD ITAB_VBAK-VBELN.

CALL TRANSACTION 'VA23' AND SKIP FIRST SCREEN.

ENDIF.

endcase.

when i double click the field itab_vbak-vbeln iam getting the transaction VA23.BUT MY PROBLEM IS

IAM NOT ENTERING INTO THE QUOTATION TO WHICH IAM DOUBLE CLICKING.iam getting MESSAGE

ENTER THE DOCUMENT NUMBER.Please help me in getting the document number in the transaction va23

when i double click the the field ITAB_VBAK-VBELN.URGENT THANKS IN ADVANCE

points will be given for good answers

regards,

Archana.