the_excerpt(); tag not working

I am having a problem with the the_excerpt(); tag which I am using in my own theme in the index.php and archive.php files. The code is below:

index.php

<?php get_header(); ?>
<section class="center-column">
<?php if (have_posts()) : ?>
    <?php while (have_posts()) : the_post(); ?>
     <!--<?php echo the_permalink(); ?>-->
    <article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>
        <header>
            <h3><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h3>
            <div class="post-date"><?php the_time(get_option( 'date_format' )); ?></div>
        </header>
        <?php the_excerpt(); ?>
        <footer class="post-footer">
            <div class="categories">Posted in: <?php the_category(', '); ?></div>
            <div class="tags">Tags: <?php the_tags(); ?></div>
        </footer>
    </article>
    <?php endwhile; ?>
    <section class="post-navigation">
        <div class="alignleft">
            <?php next_posts_link('&laquo; Older Entries') ?>
        </div>
        <div class="alignright">
            <?php previous_posts_link('Newer Entries &raquo;') ?>
        </div>
    </section>
<?php else : ?>
    <article>
        <h3>Not Found</h3>
        <p>Sorry, but you are looking for something that is not available</p>
    </article>
<?php endif; ?>
</section>
<?php get_sidebar(); ?>
<?php get_footer(); ?>

archive.php

<?php get_header(); ?>

<section class="center-column">
    <!-- Archive Heading -->
    <section>
        <h3 id='archive-heading'>
            <?php if (is_category()) { ?>
            Archive for the '<?php single_cat_title(); ?>' Category
            <?php } elseif (is_tag()) { ?>
            Posts Tagged '<?php single_tag_title(); ?>'
            <?php } elseif (is_month()) { ?>
            Archive for <?php echo get_the_date('F Y'); ?>
            <?php } ?>
        </h3>
    </section>
    <!-- End Archive Heading -->

    <!-- Post Excerpt -->
    <?php if (have_posts()) : ?>
        <?php while (have_posts()) : the_post(); ?>
            <article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>
                <header>
                    <h3><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h3>
                    <div class="post-date"><?php the_time(get_option( 'date_format' )); ?></div>
                </header>
                <?php the_excerpt(); ?>
                <footer>
                    <div class="categories">Posted in:
                        <?php the_category(', '); ?>
                    </div>
                    <div class="tags">Tags:
                        <?php the_tags(); ?>
                    </div>
                </footer>
            </article>
        <?php endwhile; ?>
    <?php endif; ?>
</section>
<?php get_sidebar(); ?>
<?php get_footer(); ?>

Sorry for the long pieces of code.

I am not sure exactly where the problem lies and would be grateful for any help you can provide.

Many thanks,
nav