Error in Posting Documents

I am trying to post documents in FBS1 transaction using the Function module "<b>POSTING_INTERFACE_DOCUMENT</b>".

However every time it is reading only the first line item and not proceeding further...

Let me know how to fix this one. thank you. Below is my code.

 loop at gt_document into wa_document.
  refresh gt_ftpost.
    perform start_posting_interface.


* ****** Header Data
    gt_ftpost-stype = 'K'.
    gt_ftpost-count = 1.
    clear: l_bldat,
           l_stodt.
    write wa_document-bldat to l_bldat mm/dd/yyyy.
    write wa_document-stodt to l_stodt mm/dd/yyyy.
    perform set_fields  using: 'BKPF-BLDAT' l_bldat,
                               'BKPF-BLART' wa_document-blart,
                               'BKPF-BUKRS' wa_document-bukrs,
                               'BKPF-BUDAT' l_bldat,
                               'BKPF-WAERS' wa_document-waers,
                               'BKPF-KURSF' wa_document-kursf,
                               'BKPF-XBLNR' wa_document-belnr,
                               'BKPF-BKTXT' 'TEST',
                               'BKPF-STODT' l_stodt,
                               'BKPF-STGRD' wa_document-stgrd.



    gt_ftpost-stype = 'P' .
    gt_ftpost-count = 1.
    perform set_fields using:  'RF05A-NEWBS' wa_document-bschl,
                               'RF05A-NEWKO' wa_document-hkont,
                               'BSEG-WRBTR'  wa_items-wrbtr,
                               'COBL-GSBER'  wa_items-gsber,
                               'COBL-PRCTR'  wa_items-prctr,
                               'BSEG-DMBTR'  wa_items-dmbtr,
                               'BSEG-ZUONR'  wa_items-zuonr,
                               'BSEG-SGTXT'  'Test'.


    gt_ftpost-stype = 'P' .
    gt_ftpost-count = 3.
    perform set_fields using:  'RF05A-NEWBS' wa_document-bschl,
                               'RF05A-NEWKO' wa_document-hkont,
                               'BSEG-WRBTR'  wa_items-wrbtr,
                               'COBL-GSBER'  wa_items-gsber,
                               'COBL-PRCTR'  wa_items-prctr,
                               'BSEG-DMBTR'  wa_items-dmbtr,
                               'BSEG-ZUONR'  wa_items-zuonr,
                               'BSEG-SGTXT'  'Test'.

    call function 'POSTING_INTERFACE_DOCUMENT'
         exporting
              i_tcode                  = pv_tcode
              i_sgfunct                = 'C'
         importing
              e_msgid                  = gv_msgid
              e_msgno                  = gv_msgno
              e_msgty                  = gv_msgty
              e_msgv1                  = gv_msgv1
              e_msgv2                  = gv_msgv2
              e_msgv3                  = gv_msgv3
              e_msgv4                  = gv_msgv4
              e_subrc                  = gv_subrc
         tables
              t_blntab                 = gt_blntab
              t_ftpost                 = gt_ftpost
              t_fttax                  = gt_fttax
         exceptions
              account_missing          = 1
              company_code_missing     = 2
              posting_key_invalid      = 3
              posting_key_missing      = 4
              record_type_invalid      = 5
              transaction_code_invalid = 6
              amount_format_error      = 7
              too_many_line_items      = 8
              company_code_invalid     = 9
              screen_not_found         = 10
              no_authorization         = 11
              others                   = 12.

    perform end_posting_interface.
  endloop.