We’ve been having some issues on the site – as we schedule a lot of our posts in advance (See – we’re organised)

Anyway – sometime this doesn’t work – and it’s very annoying both for us and our web site visitors.

So here’s why :

It all comes down to these little things called cron jobs.

Cron jobs are tasks that are performed automatically in intervals without you having to lift a finger. The program just has to be setup and the cron job handles the rest. They basically take care of repetitive and mennial tasks that can be a time-suck for humans. So we let computers handle it.

One of the best examples of cron jobs in action is daily backups of your site (which you should absolutely be doing by the way.) You don’t manually go in and backup every file. It’s done automatically through cron jobs.

You probably have several cron jobs running on your site that you aren’t even aware of. A lot of the automated tasks that you probably don’t think twice about are done through cron jobs. But most of us non-developer bloggers use plugins that set it all up, so we don’t have to know the ins and outs of how they work.

Well, the scheduled post feature of WordPress is also run by cron jobs. And unfortunately, sometimes WordPress can be finicky and the tasks misfire.

Source : http://dominiquej.com/wordpress-missed-schedule-error-fix/

This is how you fix it :

https://en-gb.wordpress.org/plugins/wp-scheduled-posts/