WP-PostRatings Not working for visitors

[Brackets removed from topic title – that’s useless and confuses things]

The post ratings plugin is not working for my visitors who have made an account at my website. They need to be logged in, but when they press the button (using the heart crystal), it just keeps loading and does nothing.
I have added the following code to my theme’s loop.php so the post ratings show up in every post:

<center><?php if (function_exists('the_ratings')) { echo expand_ratings_template('<span class="rating">%RATINGS_IMAGES% (<strong>%RATINGS_USERS%</strong> diamonds)', get_the_ID()); } ?></center>