SMS message to one particular recipient not being received

[SeasideResort]

I have an iPhone 11.  I have no issues messaging with anyone other than one person.  With that one - and only one - recipient (who also has an iPhone, 13), she does not receive  any green (i.e. SMS) messages from me ... and only me.  The first question I have is why they are even green in the first place, as one message will be blue (iMessage) and five minutes later the next message will be an SMS message, which she doesn't ever receive, even when neither of us have done anything that we can think of that could affect the behavior (like, wifi temporarily goes unavailable, etc.).  I doubt that it is a provider issue but I am running out of troubleshooting options, so here I am!

HELP!!!

[Phil_Adelphus]

@SeasideResort   In Settings - Messages - Send & Receive, do both items have the phone number checked as well as email?  Check this for both of you, I'm not sure why it would be intermittent but just as a process of elimination.

[BKNS27]

@SeasideResort 

Probably not the issue but check if she enabled RCS on her iPhone 13?

Go to Settings>Apps>Messages and scroll down to the RCS setting.

[umnikke8]

To start with, you have 2 problems in one there. First off, if you both have iPhones it's trying to use blue iMessages, but failing. That could be because one or both of you isn't properly registered to the iMessage network, or that one or both don't have a working data or wifi connection. If either of you can iMessage other people then you can narrow done which side might be having problems. You can look up other troubleshooting steps. Secondly, when the iMessage send fails, it's falling back to green SMS, but that too fails to go through. Again, you'll have to troubleshoot both sides to see if it's a problem sending or receiving.