Directly using pure JWPlayer JS (but NOT WP Plugin)

You need to properly enqueue your script. You should hook your script to the

function enqueue_custom_script() {
     wp_register_script('myJW',  get_bloginfo('template_url') . '/jwplayer.js');
    wp_enqueue_script('myJW');
}

add_action( 'wp_enqueue_scripts', 'enqueue_custom_script' );