Two days after the short message was sent, the display message failed to be sent. What's going on here?

That may be because the other party didn't receive it, and it may have been sent after restarting the phone. Sometimes, for example, after crossing the tunnel, it may always be like this. My previous mobile phone was root's, and then I turned off the self-start of the message and couldn't send it all the time. Then I turned it on.