Storing to session problem

What should I do when the session doesn't save the input I give most of the time (and by most of the time, I mean it accepts and stores the variable, sometimes it doesn't.

function to get the value input by the user

$("#submit-bttn").click(function () {
    var customPayment = $('#custom-payment').val();
    customPayment = parseFloat(customPayment);

    var test = $('#GrandTotalPrice').text();
    test = test.replace(/,/g,'');
    test = parseFloat(test);
    saveLoanCustomPaymentState(customPayment);
    selectedPlan(+((customPayment/test).toFixed(2)));

   /* console.log("test" + customLoanDownPayment);
    console.log("test" + downpaymentPlan);*/
});

ajax function to send data to session

function saveLoanCustomPaymentState(customPayment) {

    customLoanDownPayment = customPayment;
    //console.log("saveLoanCustomPaymentState" + customLoanDownPayment);
    $.ajax({   
        headers:        { 'X-CSRF-TOKEN': $('meta[name="csrf-token"]').attr('content') },
        method:         "post",
        data:           {
                            customPayment: customLoanDownPayment
                        },
        url:            '{{ url('queue/edit/save-loan-custom-payment-state')}}',
        dataType:       'json',

        success : function(customPayment) {             
            customLoanDownPayment = customPayment;
        }, {{--  end success function --}}

        {{-- error function is common to all ajax calls --}}
        @include('layouts.include.ajax-error.script')

    }); {{--  end ajax --}} 

} {{-- end function --}}

controller

 public function saveLoanCustomPaymentState() {

        //variable declarations
        $customPayment = Input::get('customPayment');
        session(['queue.loan.custom-payment' => $customPayment]);
       // $customPayment = Session::get('queue.loan.customPayment');

        return Response::json($customPayment);
    }

declaration in the controller

$selectedCustomPayment          = Session::get('queue.loan.custom-payment');

blade line that calls from the session

customLoanDownPayment = '{{ isset($selectedCustomPayment) ? $selectedCustomPayment : null }}';

sometimes it stores the value, sometimes it doesn't. what causes such behavior?