Occasionally, there is a very slight chance that a scheduled message will be delayed for a few minutes due to system maintenance or a brief system overload. In the rare instance that your message is not sent at precisely the desired time, Gremln will post it at the next available opportunity.
Another reason may include a social network error, meaning the designated social network the post was scheduled for may have stopped the post going out for reasons that violate their posting rules.
For example, Twitter has specific policies in relation to abuse and spam. When you click to view the post that hasn't gone out, you will be able to see an error message that will give you an inclination as to why the post wasn't allowed to be sent.