I had the same problem with the default design. By this i mean that when i click on a post, to make some comment, the sidebar doesn’t appear.
Here is what i’ve done to make it more user friendly (showing the sidebar). Please note that this works for the default design.
1/ make the sidebar appear
in your admin menu / appearance / editor, select your template (default) and then single post (single.php).
At the bottom of this file, just before this line : <?php get_footer(); ?>
add this one : <?php get_sidebar(); ?>
2/ now you have to make your design on two columns :
on the same file but at the top you’ll find this line : <div id="content" class="widecolumn" role="main">
replace it by this one : <div id="content" class="narrowcolumn" role="main">
hope it will help.
