Report Selection screen parameters issue

Hi,,

I am creating a custom report.

I have 2 parameters PLANT AND SERIAL NUMBER. I have a custom table and their I am keeping the serial number respect to plant.

My requirement is : I am entering plant and when I am pressing F4 on serial number, the serial numbers saved with repect to plant should come by pup up and user will select the number. Below is the code....

SELECTION-SCREEN : BEGIN OF BLOCK B1 WITH FRAME TITLE TEXT-001.

PARAMETERS :  P_PLANT TYPE WERKS_D,"Z45A_CHALLAN-PLANT,

               P_SRNO  TYPE Z45A_CHALLAN-SRNO.

SELECTION-SCREEN : END OF BLOCK B1.

AT SELECTION-SCREEN ON VALUE-REQUEST FOR P_SRNO.

   PERFORM GET_SRNO_F4.

FORM GET_SRNO_F4 .

   IF GT_DELETE IS INITIAL.

     DATA : RETURN_TAB TYPE TABLE OF DDSHRETVAL WITH HEADER LINE.

     REFRESH : GT_DELETE.

     SELECT RGP_NO PLANT ZYEAR SRNO

       FROM Z45A_CHALLAN

       INTO TABLE GT_DELETE

       WHERE PLANT = P_PLANT.

     IF  SY-SUBRC IS INITIAL .

       SORT GT_DELETE.

       DELETE ADJACENT DUPLICATES FROM GT_DELETE COMPARING ALL FIELDS.

     ENDIF.

     CALL FUNCTION 'F4IF_INT_TABLE_VALUE_REQUEST'

       EXPORTING

*       DDIC_STRUCTURE         = ' '

         RETFIELD               = 'SRNO'

*       PVALKEY                = ' '

         DYNPPROG               = SY-CPROG

         DYNPNR                 = SY-DYNNR

         DYNPROFIELD            = 'P_SRNO'

*       STEPL                  = 0

*       WINDOW_TITLE           =

*       VALUE                  = ' '

         VALUE_ORG              = 'S'

*       MULTIPLE_CHOICE        = ' '

*       DISPLAY                = ' '

*       CALLBACK_PROGRAM       = ' '

*       CALLBACK_FORM          = ' '

*       CALLBACK_METHOD        =

*       MARK_TAB               =

*     IMPORTING

*       USER_RESET             =

       TABLES

         VALUE_TAB              = GT_DELETE

*       FIELD_TAB              =

        RETURN_TAB             = RETURN_TAB

*       DYNPFLD_MAPPING        =

*     EXCEPTIONS

*       PARAMETER_ERROR        = 1

*       NO_VALUES_FOUND        = 2

*       OTHERS                 = 3

               .

     IF SY-SUBRC <> 0.

* Implement suitable error handling here

     ELSE .

       LOOP AT RETURN_TAB.

         P_SRNO = RETURN_TAB-FIELDVAL.

       ENDLOOP.

     ENDIF.

   ENDIF.

ENDFORM.                    " GET_SRNO_F4

ISSUE IS :

I am entering plant and serial number manually and doing f8 (without pressing f4 help on serial number and also not pressing press ENTER), then I am getting my plant in p_plant while checking in debug.

But when i am enter the plant and press f4 on serial number without pressing ENTER, I am not getting my p_plant value . Checked in debug.

if press ENTER after entering plant and then doing f4 ,then I am getting the p_plant value. while check in debug.

Could you please suggest why this happens and how i ll resolved the same ????

Thanks..