Dear nau k,
recently i faced same problem and here is the solution:
the following form came with default if condition IF fval IS NOT INITIAL
when sending ' ' or space to check box it will not be included in bdcdata table so commenting
the if condition solve the problem.
FORM bdc_field USING fnam fval.
* IF fval IS NOT INITIAL.
CLEAR bdcdata.
bdcdata-fnam = fnam.
bdcdata-fval = fval.
APPEND bdcdata.
* ENDIF.
ENDFORM.