Sent Delivered Opened Clicked what is the difference?

I have an automation on right now.  34 DMs sent and only 9 delivered 100% opened and clicked. Some of my followers have not had the DMs delivered to them so it’s not even a case of them not being connected to me. That’s a significant amount of undelivered DMs and it is defeating the purpose of having Manychats. What could be causing this? This is also in response to my FB ads so this is money going down the drain.  I hope someone can help