Wild guess, if it is not the CSS then check the html structure
If you have 15 other that are working with the same CSS, then check if the html structure is the same for the other as well or not, since your html is well commented you should be able to spot it easily.
You may try to put the sidebar div inside container
Now it is:
</div><!-- #container -->
<div id="primary" class="widget-area" role="complementary">
.......
</div>
</div><!-- #main -->
Try to move the </div><!-- #container -->
like:
<div id="primary" class="widget-area" role="complementary">
.......
</div>
</div><!-- #container -->
</div><!-- #main -->
You may need to tweak a template or something to do it