Wordpress has a wonderful feature allowing you to schedule posts to publish at a future date. This is perfect for when you'll be on vacation or you need a post to go online at an exact time while you're away from your computer.
Unfortunately Wordpress has had a long term issue with being inconsistent about this functionality always working. Some users have never experienced an issue while others have it consistently miss scheduled posts.
If you experience this issue there is a plugin that can help assist with checking for any missed posts. It's called Scheduled Post Trigger and will check your database for any missed posts each time a visitor loads your blog (not on admin visits).
To install Scheduled Post Trigger
Login to your Wordpress admin and go to Plugins > Add New
Search the plugins database for "Scheduled Post Trigger"
Click to Install the plugin. Then to Activate it.
That's all...it will start working in the background.
Hopefully in the future the Wordpress Core developer teams will be able to find a solution that resolves this issue for all users. Until that comes it's nice to know that there are options like the plugin above.