The most common reason for this is that your WordPress theme doesn’t include the required WordPress functions which are used by plugin’s to add their files to your page. These functions are the wp_head function – which should be in your theme’s header.php file – and the wp_footer function – which should be in your footer.php file. These functions are required by all WordPress plugins in order for them to be able to add files to your page.
To add them to your theme, just do the following:
1) Open your theme’s header.php file and add the following immediately before the closing
<?php wp_head(); ?>
2) Open your theme’s footer.php file and add the following immediately before the closing
<?php wp_footer(); ?>