I am trying to send a Group MMS text. There are 6 recipients. I hit "send" and the progress bar goes almost to the very end, and then hangs up....and eventually I get a "Not Delivered" error message. I click on the "i" icon and it says "your message was not sent. Tap try again to send this message." I've done that time and time again, and the same thing happens. What am I doing wrong?