User Photo – Not All User Photos Displaying

I’ve got the User Photo plugin working to display the circular .png user images in most cases (without the transparent image background displaying black).
However I have an issue that some user photos are not displaying at all.
Below is the line of code I am using to display the user photos on the same page (list of posts):

<p class="staff-avatar"><a href="/index.php?p=<?php echo $post->ID; ?>"><span class="aboutIcons"><?php echo userphoto_the_author_photo(); ?></span></a></p>

When the source code of the browsed page is inspected in the instances where the images are not displayed the code:
<?php echo userphoto_the_author_photo(); ?>
appears to have been completely omitted/ignored.
Below is an example of the code returned where the user photos are displayed as expected:

<p class="staff-avatar"><a href="/index.php?p=211"><span class="aboutIcons"><img src="http:/website.com/wp-content/uploads/userphoto/3.png" alt="User Name" width="140" height="140" class="photo" /></span></a></p>

However, in the instances where the user photos are not displayed the following code is returned:

<p class="staff-avatar"><a href="/index.php?p=217"><span class="aboutIcons"></span></a></p>

It appears something has changed somewhwere that is affecting all newly created users. The version of WordPress has remained unchanged since the original installation. All the originally setup users display their photos. I’ve inspected the wp_usermeta table and all the data is consistent across all the users. All the image files are consistently named n.png and n-thumbnail.png too. However there is no indication as to how the plugin is delivering the paths to the image files.
Also I am wondering if there is a limit to the number of user photos that the plugin can handle?
Any assistance would be greatly appreciated.