The server cache stores the script output and sends it back to users. It'll make your website faster. But, it also stops FastCron from running your cronjobs.

To bypass the server cache, you can add the __random__ keyword to your cronjob URL like this https://example.com/cron.php?__random__
or
https://example.com/cron.php?name=value&__random__

FastCron can do it for you automatically.
Just visit the Account settings page, and check the box Automatically update cronjob URL with __random__ to bypass cache.

The next time your cronjob executes, FastCron will add the __random__ keyword if it detects a server cache.