Search box working only for post title. Maybe because of custom posts?

In my site running WP 3.0.4 I created a new taxonomy for custom posts (with Easy post types plugin) and some custom fields.
In my homepage I show only the title and the permalink of the custom posts and also the thumb of an image.
I put a search box but I get results only if I search for string that are in the titles, nothing for any string that is in the posts but not in the title.
This is the search box code in the header:

<form method="get" id="searchform" action="<?php bloginfo('home'); ?>/">
<div><input type="text" size="18" value="<?php echo wp_specialchars($s, 1); ?>" name="s" id="s" />
<input type="submit" id="searchsubmit" value="Ricerca" class="btn" /></div></form>

Does anyone got any idea to fix this problem?