colspan sorting problem - last column not sortable after adding a column

Hi, everyone.
I don't think it's a bug of datatable. But I still hope to get a workaround for this weird problem. Here is the thing.
My table definition is

<table id="myTable" class="table table-striped table-bordered" border="0">
                        <thead>
                            <tr>
                                <th rowspan="2" id="th1">th1</th>
                                <th rowspan="2">th2</th>
                                <th rowspan="2">th3</th>
                                <th rowspan="2">th4</th>
                                <th rowspan="2">th5</th>
                                <th colspan="5">th6</th>
                            </tr>
                            <tr>
                                <th>th6_1</th>
                                <th>th6_2</th>
                                <th>th6_3</th>
                                <th>th6_4</th>
                                <th>th6_5</th>
                            </tr>
                        </thead>
                        <tbody>                        
                        </tbody>
                    </table>

In the javascript code, i added a column dynamically to be the first column of the table, including the header, let's say

<th rowspan="2">new_th1</th>

And don't worry, I also added the corresponding data to that newly added column, something like this

<td> little foxy thing</td>

Then the last column, i.e <th>th6_5</th> is not sortable, I mean, you can not click the header of that column to do a sortion, event it's not clickable. So any thoughts how to figure it out?

Thanks