Why is it easily possible to theme node add forms and not node edit forms?
Have you got the code in your template.php file set up correctly?

It should look like this:

function THEMENAME_theme($existing, $type, $theme, $path) {
    return array(
        'FORMNAME' =&gt; array(
           'arguments' =&gt; array('form' =&gt; NULL),
           'template' =&gt; 'page--node--add--mycontenttype.tpl.php',
          'render element' =&gt; 'form',
          ),


You can find FORMNAME, by viewing the source of the page and looking for the id of the form.