I own the S8 and have also had this issue. The best suggestion would be to do a restart of the phone as it is possible that the messages may have gotten stuck in sending between the phone and the main servers.
I've notices this in my note note 9 I'm on Rogers. I'm starting to think it has to do with rcs which is still a little buggy at the moment. What seems to work if you turn wifi on and off.
