Three services, one app

Messages juggles iMessage (blue bubbles), plus RCS and SMS (green). Which one your message is riding on often explains why it stalled.

“the iPhone's Messages app is more complex than it seems”

Just tap and try again

Hit the red exclamation point next to the failed message and choose to send it again. A second attempt clears a surprising number of failures.

No signal, no delivery

iMessage and RCS both need the internet, and SMS needs decent reception. Flip airplane mode on, then off, to force your iPhone back onto the network.

Send it as a text

Press and hold the failed message, then pick Send as Text Message. Turn on Retry as Text Message in Settings > Apps > Messages to make that automatic.

“Your blue bubble becomes green and is sent over the cell network”

Check iMessage is actually on

Switching iPhones sometimes leaves iMessage unactivated. Toggle it off, wait a minute, turn it back on, and confirm the right number or email under Send & Receive.

Maybe it isn't you

Apple's servers go down occasionally, so check the System Status page. Also try a different contact: if only one person never gets your texts, their phone may be off or you may be blocked.

Reboot, then update

Restart the phone first. Then go to Settings > General > Software Update, since outdated iOS handles online services badly.

“older software that isn't maintained is more likely to have trouble with online services”