Imessage no delivery notification

For some reason, when I send an Imessage to one contact, it never shows the message as delivered. It works for other Imessage users, and worked up until about an hour ago. I received a message from them, and then when I went to reply, it never showed "delivered" under the message. While this obviously isn't a huge deal, I want to make sure that they are in fact receiving my message. I guess I've come to count on the delivered notification more than I realized. Like I said, it works for other users, as well as I've turned the phone off and on several times. there have been times where there has been a delay, but it has been at least half an hour since I sent the first message. I was able to send one as a regular text message that I know they got, but it's just irritating as to why all of the sudden this is acting up and possibly just for this one contact. I haven't changed any message settings or anything, so I don't think that is the problem. Does anyone know what could be causing this and how I can fix it?