У меня не работают кнопки далее и назад

session_start();
include "header.php";
?>
        <div class="row" style="margin: 0px; padding:0px; margin-bottom: 50px;">

            <div class="col-lg-6 col-lg-push-3" style="min-height: 500px; background-color: white;">
                <!--start editare-->
                <br>
                <div class="row">

                    <br>
                    <div class="col-lg-2 col-lg-push-10">

                        <div id="current_que" style="float:left">0</div>
                        <div style="float:left">/</div>
                        <div id="total_que" style="float:left">0</div>
                    </div>
                    <div class="row" style="margin-top: 30px">

                        <div class="row">
                        <div class="col-lg-10 col-lg-push-1" style="min-height: 300px; background-color: white" id="load_questions">

                        </div>
                    </div>
                </div>
                <div class="row" style="margin-top: 10px">
                    <div class="col-lg-6 col-lg-push-3" style="min-height: 50px">
                        <div class="col-lg-12 text-center">                         
                          <input type="button" class="btn btn-warning" value="Previous" oneclick="load_previous();">&nbsp;
                          <input type="button" class="btn btn-success" value="Next" oneclick="load_next();">
                        </div>
                    </div>
                </div>
            </div>
            <!-- Sfirsit editare-->

       </div>

    </div>
<script type="text/javascript">
    function load_total_que()
    {
        var xmlhttp=new XMLHttpRequest();
        xmlhttp.onreadystatechange=function() {
            if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
                document.getElementById("total_que").innerHTML=xmlhttp.responseText;
            }
        };
        xmlhttp.open("GET","forajax/load_total_que.php",true);
        xmlhttp.send(null);
    }
    var questionno="1";
    load_questions(questionno);
    function load_questions(questionno)
    {
        document.getElementById("current_que").innerHTML=questionno;
        var xmlhttp=new XMLHttpRequest();
        xmlhttp.onreadystatechange=function() {
            if (xmlhttp.readyState == 4 && xmlhttp.status == 200) 
            {
                if(xmlhttp.responseText=="over")
                {
                    window.location="result.php";
                }
                else{
                    document.getElementById("load_questions").innerHTML=xmlhttp.responseText;
                    load_total_que();
                }
            }
        };
        xmlhttp.open("GET","forajax/load_questions.php?questionno="+ questionno,true);
        xmlhttp.send(null);
    }
    function load_previous()
    {
        if (questionno=="1") 
        {
            load_questions(questionno);
        }
        else
        {
            questionno=eval(questionno)-1;
            load_questions(questionno);
        }
    }

    function load_next()
    {
            questionno=eval(questionno)+1;
            load_questions(questionno);
    }
</script>


<?php
include "footer.php"; 
?>

Я не знаю в чем дело, может я чего-то пропускаю, помогите пожалуйста !