I have a page with many items pulled from posts.

I have it set up to only display 10 posts at a time, but my previous/next button isn't actually displaying the next or previous posts - it justs keeps displaying the same posts.

Here's the function I wrote:

  function add_shop() { 
    if (is_page('shop') || is_page('7')) { ?>
  <div id="content">
    <div class="post_box">
      <div id="column_1">
        <div id="shop_wrapper">
          <?php query_posts('tag=shop&orderby=title&order=ASC&posts_per_page=10');

if (have_posts()) : ?>
          <?php while (have_posts()) : the_post(); ?>
          <div class="shop_item"> <a href="<?php getCustomField('link_to_project_page'); ?>"><img src="<?php getCustomField('shop_thumbnail_image'); ?>" alt='photo of <?php getCustomField('title'); ?>' class="shop_thumb" /></a>
            <div class="shop_content">
              <h4><a href="<?php getCustomField('link_to_project_page'); ?>">
                <?php getCustomField('title'); ?>
                </a></h4>
              <?php getCustomField('duration'); ?>
              <?php getCustomField('paypal_code'); ?>
            </div>
          </div>
          <?php endwhile; ?>
        </div>
        <?php posts_nav_link(); ?>
      </div>
      <?php else : ?>
      <h2>Not Found</h2>
      <p>Sorry, but you are looking for something that isn't here.</p>
      <?php include (TEMPLATEPATH . "/searchform.php"); ?>
      <?php endif; ?>
    </div>
  </div>
  <div id="sidebars">
    <div id="sidebar_1" class="sidebar">
      <ul class="sidebar_list">
        <li class="widget">
          <div class="widget_box">
            <?php dynamic_sidebar(5); ?>
          </div>
        </li>
      </ul>
    </div>
  </div>
  <?php }   }