BAPI_CONTRACT_CHANGE is not working with extension fields

My Custom requirement is to add a field in contract item level (Me31k/ME32k/Me33k).

To achieve this I have done below steps and working fine.

But My other requirement is to update the field using bapi_contarct_change depending on some logic.

And then my code is not working.

FOR MORE DETAILS  GET BACK TO ME.

Code for updating the zfield is below,

REPORT  ZTEST_PO_ITEM_CHANGE.


data:   lt_item              typetableof bapimepoitem,
str_item              type bapimepoitem,
lt_itemx              typetableof bapimepoitemx,
str_itemx            type bapimepoitemx,
lt_extensionin        typetableof bapiparex WITHHEADERLINE,
str_extensionin      type bapiparex,
lt_extensionout      typeTABLEOF BAPIPAREX,
str_bapi_te_mepoitem  type bapi_te_mepoitem,
str_bapi_te_mepoitemx type bapi_te_mepoitemx,
return_t type  BAPIRET2_T,
ebelp type ebelp,
ebeln type ebeln.


ebelp = '00020'.
ebeln = '4600000004'.


clear: lt_item[], lt_itemx[], lt_extensionin[].


str_item-po_item = str_itemx-po_item = ebelp.
append str_item to lt_item.
append str_itemx to lt_itemx.

*------------------------*
*-New Z-fields in EKPO  -*
*------------------------*

CALLFUNCTION'CONVERSION_EXIT_ALPHA_INPUT'
EXPORTING
input  = ebelp
IMPORTING
OUTPUT = ebelp.

str_bapi_te_mepoitem-po_item     = ebelp.
str_bapi_te_mepoitem-zzlc_no  = '123456789123'.


str_bapi_te_mepoitemx-po_item    = ebelp.
str_bapi_te_mepoitemx-zzlc_no    = 'X'.



move'BAPI_TE_MEPOITEM'to str_extensionin-structure .  "str_extensionin-structure        = 'BAPI_TE_MEPOITEM'.
move str_bapi_te_mepoitem to str_extensionin-valuepart1. "str_extensionin-valuepart1(114)  = str_bapi_te_mepoitem.
append str_extensionin to lt_extensionin.

CLEAR:str_extensionin.

move'BAPI_TE_MEPOITEMX'to str_extensionin-structure.      "str_extensionin-structure        = 'BAPI_TE_MEPOITEMX'.
move str_bapi_te_mepoitemx to str_extensionin-valuepart1.  "str_extensionin-valuepart1(12)   = str_bapi_te_mepoitemx.
append str_extensionin to lt_extensionin.

clear return_t[].
CALLFUNCTION'CONVERSION_EXIT_ALPHA_INPUT'
EXPORTING
input  = ebeln
IMPORTING
OUTPUT = ebeln.

*  call function 'BAPI_PO_CHANGE'
*    exporting
*      purchaseorder = ebeln
*    tables
*      return        = return_t
**     poitem        = lt_item
**     poitemx       = lt_itemx
*      extensionin   = lt_extensionin.

CALLFUNCTION'BAPI_CONTRACT_CHANGE'
EXPORTING
purchasingdocument          = ebeln
*     HEADER                      =
*     HEADERX                     =
*     VENDOR_ADDRESS              =
*     TESTRUN                     =
*     TECHNICAL_DATA              =
*   IMPORTING
*     EXP_HEADER                  =
TABLES
*     ITEM                        = lt_item
*     ITEMX                       = lt_itemx
*     ACCOUNT                     =
*     ACCOUNTPROFITSEGMENT        =
*     ACCOUNTX                    =
*     DELIVERY_ADDRESS            =
*     ITEM_COND_VALIDITY          =
*     ITEM_COND_VALIDITYX         =
*     ITEM_CONDITION              =
*     ITEM_CONDITIONX             =
*     ITEM_COND_SCALE_VALUE       =
*     ITEM_COND_SCALE_QUAN        =
*     ITEM_TEXT                   =
*     HEADER_TEXT                 =
*     HEAD_COND_VALIDITY          =
*     HEAD_COND_VALIDITYX         =
*     HEAD_CONDITION              =
*     HEAD_CONDITIONX             =
*     HEAD_COND_SCALE_VAL         =
*     HEAD_COND_SCALE_QUAN        =
*     PARTNER                     =
*     PARTNERX                    =
*     RELEASE_DOCU                =
EXTENSIONIN                 = lt_extensionin
*   EXTENSIONOUT                = lt_extensionout
RETURN                      = return_t
.

data: ls_BAPIRET2 type BAPIRET2.
CALLFUNCTION'BAPI_TRANSACTION_COMMIT'
EXPORTING
WAIT   = 'X'
IMPORTING
RETURN = ls_BAPIRET2.

But the above mentioned code is not working. My zfield is not getting updated.

Please help it is urgent...........thanks in advance!