These days I had some problems with a lightbox plugin for one of my Wordpress blogs.
I used the Wordpress plugin for the Shadowbox on many sites before and I was very surprised why it wasn't worked.
So it seems that the jQuery library was not available inside the HTML head section.
I tried a lot of things until I used the same theme on a blog where the plugin has worked for more than a year.
The result; that the Lightbox doesn't show up. Next I compared both themes and noticed that the wp_footer() function was missing inside the footer template file.
I added the function (you need that one for the Wordpress stats plugin as well) and yes the missing jQuery problem was solved.
ADD ALWAYS the function wp_footer(); into your themes footer (right before the body close tag)