This is more than likely do to the number of users. You’re probably (only guessing) either exceeding time limists or exceeding memory limits. Or there could be some type of PHP error that’s causing the AJAX request to fail.
You need to do some debugging and figure out what’s causing the issue. You’ll need to use error logging so you can see what’s happening during the AJAX request. https://codex.wordpress.org/WP_DEBUG
