iMessage not working with selective iPhone contacts!

I updated my phone to IOS 9.0.2 the day the update was released and haven't had any problems. All of the sudden, (as of last night) iMessage has stopped working for a few of the people I send messages to. I have turned iMessage on/off, reset my network settings, restarted the phone, and signed in an out of iCloud to reactivate iMessage. I don't know what else to do or what is causing the problem. Why would iMessage stop working with some of my fellow iPhone users but then seem to work fine when I message other iPhone users? Please help!