Web evoting app中的下一个按钮出现故障

I'm trying to build a web e-voting app with the usage of next and previous button, but whenever I click on the next button, the next div is not shown.

Here is my php jquery code

       <div class="vote_div">

        <form id="voting_form" method="post">

        <?php

        $post_query = $db_handle->prepare("SELECT DISTINCT `post_id` FROM `aspirants` ORDER BY `post_id`");
        $post_query->execute();
        $count_post = $post_query->rowCount();

        if($count_post == 0) 
        {
            echo error_admin_msg("No aspirant yet.");
        }
        else
        {
            $fetch_post = $post_query->fetchAll();
            $i = 1;

            foreach($fetch_post as $key=>$row) 
            {

               ?>

                <div id="vote_div<?php echo $i;?>" class="all_vote_div">

                <?php

                $post_id = $row['post_id'];
                $post_name = get_post_name($post_id);

                $query = $db_handle->prepare("SELECT *FROM `aspirants` WHERE `post_id` = ? AND `qualify` = '1'");
                $query->bindparam(1,$post_id);
                $query->execute();
                $count = $query->rowCount();

                if($count == 0) 
                {
                    echo error_admin_msg("No aspirant yet for the post of <b>$post_name </b>.");
                }
                else
                {
                    ?>

                    <h3 align='center'>POST OF <?php echo strtoupper($post_name);?></h3>

                    <?php

                    $fetch_aspirants = $query->fetchAll();

                    foreach($fetch_aspirants as $fetch) 
                    {
                        $aspirant_id = $fetch['aspirant_id'];
                        $unique_id = $fetch['unique_id'];
                        $aspirant_student_id = $fetch['aspirant_student_id'];
                        $aspirant_fullname = get_student_fullname($aspirant_student_id);
                        $aspirant_nickname = $fetch['nickname'];
                        $path = $fetch['path'];

                        if($path == "")
                        {
                            $folder = "../images/";
                            $full_path = $folder."default.png"; 
                        }
                        else
                        {
                            $full_path = "../aspirants/".$path;
                        }

                        ?>

                        <div style='clear:both'></div>
                            <div class="col-md-4 col-sm-4">

                                <img src="<?php echo $full_path;?>" alt="<?php echo $aspirant_fullname;?>" class="img-responsive img-circle"/> <br/><br/><br/>
                            </div>

                            <div class="col-md-8 col-sm-8">
                                <h3> Fullname &raquo; <?php echo $aspirant_fullname;?> </h4><br/>

                                <h4> Nickname &raquo; <?php echo $aspirant_nickname;?> </h4><br/>

                                <input type="radio" id="post<?php echo $aspirant_id;?>" name="<?php echo $aspirant_id;?>" value="<?php echo $aspirant_id;?>"/> Vote

                                <input type="radio" checked='checked' style='display:none' value="none" id="post<?php echo $aspirant_id;?>" name="<?php echo $aspirant_id;?>"/>

                            </div>
                    <?php              
                    }
                }

                if($i == 1)
                {
                    ?>
                    <div align="center">
                        <button id='<?php echo $i;?>' class='next btn btn-lg btn-success' type='button'> Next &raquo;</button>
                    </div> 
                <?php
                }
                else if($i > 1 || $i < $count_post)
                {
                    echo "ppp";
                    ?>
                    <div align="center">
                        <button id='<?php echo $i;?>' class='previous btn btn-lg btn-success' type='button'> Prev &laquo;</button> &nbsp; &nbsp;

                        <button id='<?php echo $i;?>' class='next btn btn-lg btn-success' type='button'> Next &raquo;</button>
                    </div> 
                <?php
                }
                else if($i == $count_post)
                {
                    ?>
                    <div align="center">
                        <button id='<?php echo $i;?>' class='previous btn btn-lg btn-success' type='button'> Prev &laquo;</button> &nbsp; &nbsp;

                        <button id='<?php echo $i;?>' class='next btn btn-lg btn-success' type='submit'><i class="icon-check"></i> Submit </button>
                    </div>  
                <?php
                }
                $i++;
            }
        }   
        ?>
        </form>
    </div>

</div> 

    <script>

        $('.all_vote_div').addClass('hide');
        $('#vote_div'+1).removeClass('hide');

        $(document).on('click','.next',function()
        {
            var last = parseInt($(this).attr('id'));     
            var nex = last+1;
            //alert(nex);
            $('#vote_div'+last).addClass('hide');
            $('#vote_div'+nex).removeClass('hide');
        });

        $(document).on('click','.previous',function()
        {
            var last = parseInt($(this).attr('id'));     
            var pre = last-1;
            $('#vote_div'+last).addClass('hide');
            $('#vote_div'+pre).removeClass('hide');
        });

    </script>

I'll be glad if you can help out. Thanks