Main Menu doesn't show up

I have created a sub theme for Zen. When I am logged in as administrator, the main menu doesn't show up at all unless I move the main menu block into a region i.e. Header. The main menu shows up behind the logo at the top of the page when I am not logged in.
I have changed the default theme to Bartik and the main menu shows up as tabs above the header.
I'm pretty new to Drupal and this is my first attempt at using a base theme/sub theme. I don't know PHP, but I do know HTML & CSS.
Any advice?
Thanks, Sheryl