Tablesorter，PHP和AJAX

I Have a problem when I try to sort my table with tablesorter.js. It looks easy to use it, but in my case it doesn't work... First of all, I explain my application:
1.- I have a database with several data
2.- I developed a web interface to search that data (HTML)
3.- I wanted to make interactive communication with the database, so I used AJAX
4.- I build the querys and the table with the results in another php file

So, in my HTML file:

<div id = "DivResults"></div>

And in the header of this file I added the jquery and tablesorter.js references and to my CSS file the contento of the CSS from tablesorter

$(document).ready(function()     {         $("#myTable").tablesorter();     } ); 

Now, in the php file I have the following code:

    <table id="myTable" class="tablesorter" style="font-size: 8pt; position:relative; top: -10px;" >
                    <thead>
                        <tr>
                            <th><b>Col1</b></th>
                            <th><b>Col2</b></th>
                            <th><b>Col3</b></th>
                            <th><b>Col4</b></th>
                            <th><b>Col5</b></th>
                        </tr>
                    </thead>
                    <tbody>

    <?php
         for ($i = 0; $i<$n;$i++)
           {
               $Result_array = ordenar_array($Result_array,2, SORT_ASC,3,SORT_ASC);
              echo "<tr>";
              for ($aux = 0; $aux<5;$aux++)
            {
               echo "<td> ".$Result_array [$i][$aux]."</td>";
            }

            echo "</tr>";
          }
    echo "</tbody>";
    echo "</table>";
    ?>

The table it's ok, the data are also ok and I display it in the correct way, but can't sort it with this function. Could anyone help me?

EDIT: this is the code that I added to my CSS file:

/* tables */
table.tablesorter {
    font-family:arial;
    background-color: #CDCDCD;
    margin:10px 0pt 15px;
    font-size: 8pt;
    width: 100%;
    text-align: left;
}

table.tablesorter thead tr th, table.tablesorter tfoot tr th {
    background-color: #e6EEEE;
    border: 1px solid #FFF;
    font-size: 8pt;
    padding: 4px;
}
table.tablesorter thead tr .header {
    background-image: url(../img/bg.gif);
    background-repeat: no-repeat;
    background-position: center right;
    cursor: pointer;
}
table.tablesorter tbody td {
    color: #3D3D3D;
    padding: 4px;
    background-color: #FFF;
    vertical-align: top;
}
table.tablesorter tbody tr.odd td {
    background-color:#F0F0F6;
}
table.tablesorter thead tr .headerSortUp {
    background-image: url(../img/asc.gif);
}
table.tablesorter thead tr .headerSortDown {
    background-image: url(../img/desc.gif);
}
table.tablesorter thead tr .headerSortDown, table.tablesorter thead tr .headerSortUp {
background-color: #8dbdd8;
}