Oh no! Your message didn't send as you planned and that has to make you frustrated. No worries, it's probably something small that can be adjusted to make it work as you desire. There are some common things that can happen that would cause a message to not go out as planned but nothing that can't be remedied.
Check to see if your message is active: If the message is not set to active, it will not send on schedule. You can tell if a message is active or not by the color of the dot to the right of the message. If it is red then it is NOT active. If green then that message is active.
Timing of when you added the Workflow Step: If the message was scheduled after a person had joined, they can't get a message set to "Upon Group Join". Always make sure to set up your Workflow BEFORE adding people to the Group. Because the Workflows operate based on when a person joins then if a person was in the Group for three days and you set up a message to send 2 days after joining then that person would not get the message as he would have been in for three days already.
Check to see if the person is opted in: If the person opted out of getting messages, they will not be sent messages. You can check this by clicking on their name in the People section. They can reply START to your number to opt-in and get messages again.
Check to see if anyone is in the Group: If no one has joined or been added to the Group, the messages won't send yet. The Workflow is based on when a person joins, if no one joins the Group, no one can get the messages. You can see who is in the Group by going to the Members tab of the Group.
You made the message in the wrong place: It's possible you are wanting a message to go at a specific time and date for all members of the Group. If that's true, you will want to do a one-time scheduled message. To learn more about the differences between Automated Workflows and Scheduled messages click here.