Where are you putting the excerpt in? In the explicit excerpt in your Write Post panel? Or are you using the <!--more-->?
http://codex.wordpress.org/The_Loop_in_Action shows you how to use the Loop in various ways to generate content and this section talks about how to make only excerpts show on the front page.
http://codex.wordpress.org/The_Loop_in_Action#Excerpts_Only
Its in my index, as
<div class=”entry”>
<?php if(is_category() || is_archive()) {
the_excerpt(‘Read more…’);
} else {
the_content();
} ?>
Does your theme come with an archive.php and/or category.php template? If so, they’ll be used for those types of queries rather than the index.php, and cause WordPress to ignore your code.
Also note the_excerpt() does not provide a parameter for a ‘Read more…’ sort of link text.
