A common issue faced by theme developers is that of the site slogan not appearing even when the variable $site_slogan is present.

The site slogan is disabled by default.  To turn it on in Drupal 6.6 go to the Theme selection list and choose Configuration.  The site slogan is one of the checkboxes available on the left-hand side.

The same thing might have to be done in the theme-specific settings to take effect.