iMessages not sending to one person?

As of yesterday, one of my contacts was not reciveing my imessages, although they said "Delieverd" at the bottom. I updated my phone, restarted my phone, did a network reset and turned off and on my imessage tab numerous times. The other person has yet to compelte the update, but has restarted and turned off imessage and turned it back on several times. Wherever I go, I make sure I have wifi so not having service cannot be the issue. The other contact has an iphone 4s and I have an iphone 4. My messages send as a plain text message (green) and the person recieves them, but I do not recieve the reply. Is there any other factors that could apply to this problem?