Double Click Valid but Single Click Invalid in ALV Grid

In my ALV grid, double click is valid, but single click is invalid. In addition, field "box" in internal table " t_outtab" always stores last value of line-selection checkbox but not current value.

Part of source code:

CALL FUNCTION 'REUSE_ALV_LIST_DISPLAY'

EXPORTING

i_callback_program = l_prog

it_fieldcat = t_fieldcat

is_layout = s_layout

is_variant = variant

it_sort = t_sort

it_events = t_events

i_callback_user_command = 'USER_COMMAND'

i_save = 'A'

TABLES

t_outtab = t_payrq_list.

FORM user_command USING ucomm LIKE sy-ucomm

selfield TYPE slis_selfield.

CASE ucomm.

*-- double click

WHEN '&IC1'.

READ TABLE t_payrq_list

INTO ls_payrq_list

INDEX selfield-tabindex.

CALL FUNCTION 'COPC_RPR_DISPLAY_REQUEST'

EXPORTING

iv_request_number = ls_payrq_list-keyno

IMPORTING

et_message = lt_messages

.

EXIT.

*-- single click

WHEN '&ETA'.

READ TABLE t_payrq_list

INTO ls_payrq_list

WITH KEY box = abap_on.

CALL FUNCTION 'COPC_RPR_DISPLAY_REQUEST'

EXPORTING

iv_request_number = ls_payrq_list-keyno

IMPORTING

et_message = lt_messages

.

EXIT.

ENDCASE.

ENDFORM.

Please help me solve this problem.