I'm developing a site that is based around a plugin like this. I looked into other plugins but for some reason none of them would work for me. The more I looked into how this works the more I noticed the word 'cron' coming up.
If you are using a linux server, there is a file in your root directory called .htaccess (the dot makes it hidden, so google how to show it). In there add the following to the end of the code.
<Files wp-cron.php> allow from all Satisfy Any </Files> # END WordPress
If you are using a Windows server, the file is called mod_access(I think).
When I made this change, the plugin which was previously unresponsive worked perfectly with every feature that was advertised. This feature of the site literally was a make or break thing for my client. So I thought I would share it encase anyone else is in the same boat!
I would hope that someone with a bit more knowledge relating to .htaccess files and the code above would advise to security issues that may arise from using it and possibly offer an alternative? For now, use it at your own risk!
Anyways, I hope this has helped anyone that might be stuck with this plugin.