Change the value of var and load php page

Question

onchange doesn’t work, it just works on the first selection, then it doesn’t. ideas ??

function getComboA(sel) {
    $(document).ready(function(){
        var id = sel.value;
        $('#frontend').bcp({
            'ID': id,
            'loadURL': 'bcp/db/carrega.php',
            'sendURL': 'bcp/db/send.php'
        });
    }) 
};