only 4 primary menu items

Hi, I used the Barron theme for making a simple website in drupal 5.x (www.spidibulle.com); which worked out perfectly.
When defining my primary menu items however, I found out that I only could use 4 items, not 5.
When adding the fifth, it displays the item text on screen, but it cannot be highlighted (and of course as such not selected). Is this because I'm a newbie to drupal and barron and doing something wrong?

Many thanks in advance for your help.