iMessage messages saying not delivered despite iMessage being turned on; Some delivering regardless, but still saying not delivered on my end

iMessage is turned on on my phone, iMessage is assumedly on on their iPhone because we often text using iMessage, yet as of late a lot of times the message will not send, my text bubble will turn green, it won’t even actually send as a text message—it just won’t send at all.

This can’t be an isolated issue, because I have another friend who I text on iMessage, and they had this problem even before I did where they would send me messages and—on their end—it shown as not sent, and their bubble turned green.

The thing is, though it said that, I still received the messages—at least some of them. And though on my end my messages are saying not sent, my other friend receives the messages. Some of them.

Is this an IOS update bug? Is there a way to fix this?