selected check box lines unable to update in custom form which is developed from template.fmb

Developing custom form from template.fmb. which is for update the Receipt transactions attribute columns, check box item added on rcv_transactions block, but when i save the form, from tool bar, all modified rows updating.

but not considered check box selected lines, while updating rows

could you please help how to develop only checkbox selected lines only need to save, after save the form.