iPhone messages never get delivered

Having problems with iMessage dropping messages between two iPhones.

The issue of iMessage losing messages sent to users who have switched to non-apple devices is well documented, but in this case we're seeing one iPhone user message another, and the message never gets through. There's no indication on the sender's side that the message failed- we just happened to notice it on our own. It's happened at least twice within the past few months (though probably a lot more)

Here's the latest example. My friend asks a question, and I respond a couple of minutes later, but she never gets it on her end- though everything looks fine on my side:

One possible clue: Her question was sent as an SMS, not an iMessage (which apparently happens when a phone can't access normal data coverage - hence the green bubble.) Did that somehow cause my subsequent reply to get lost?

Whatever the case, there's no technical excuse for a queued messaging system to be silently vanishing messages- Apple's got some explaining to do.