I am using code like this to replace a string in my posts.

function replace_text($text) {
$text = str_replace('look-for-this-string', 'replace-with-this-string', $text);
$text = str_replace('look-for-that-string', 'replace-with-that-string', $text);
return $text;
}
add_filter('the_content', 'replace_text');
add_filter('the_excerpt', 'replace_text');

This is working great, except on my archive index pages. On these pages, the text is not being replaced. What am I missing?