Why does my iPhone say text message “not delivered” when sometimes they are delivered

It could be your internet. Another way ti fix this is try restarting. Or go to settings general date and time and make sure its set to the automatic time.

Ask your carrier.

I have been having a similar issue since the last update. I reached out to my carrier as step 1. They did trouble shooting, got me a new SIM card, etc. It continued to happen without no pattern with 2-4 bars so my carrier now has me working with a tech at Apple.