Why do I keep getting 'Not Delivered' messages on my iPhone?

If these are group messages, it is possible that only one recipient is not receiving the message. Check individually with the recipients and see if someone has received the message. It could be that one of the recipients is out of contact, etc. I've seen this in the past, but not very often. Does this happen with groups that are all iPhones, or are they with groups that contain non-iPhone devices? It is more difficult to troubleshoot messages that contain non-iOS devices because they involve SMS/MMS that is a carrier responsibility. For those it could be your carrier or the other person's carrier. Make sure your settings in Messages are all turned on, including the Send as Text Message switch, MMS Messaging, and RCS Messaging.

Interesting, since you mention multiple people, sounded like you were referring to group messages. Like I mentioned in my post, I've seen these messages and that is why I had mentioned checking with people since many times the messages are going through. It can be a number of reasons on either end when it comes to iMessage. Any disturbance in the connection can cause it. If it continues on a more regular basis, you can sign out of iMessage, then restart your device, then sign back in again and see if that helps.

I also didn't ask, did you tap on the ! to attempt to deliver a second time? Did it go through that time?

I am experiencing similar issues. iPhone will fail to send photos or audio messages to iPhone users or Android users. I can be on or off wifi networks. I have no problem sending media and messages through Signal though.

AT&T is my carrier. A friend has similar issues but is on T Mobile.

Starting in May, individual messages to my wife are not going through to her iPad. Every attempt has met with a Not Delivered! Very frustrating, as she's on the same home network and is part of the family (just the two of us) plan for iCloud storage. And why, when you go into Messages settings, is iMessage still being referenced?