The excerpts are hard coded in the theme for homepage (index.php), archives (archive.php) and search (search.php) pages for the posts. To change them to use <!--more--> tag so that you can have more control over the post content, please follow the steps below:
Replace :
<?php echo str_replace('[...]', '', get_the_excerpt()); ?>
<a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>">Keep Reading →</a>with:
<?php the_content('Keep Reading →'); ?>