Same problem here with mine. And it doesn't matter whether the message is long or short. I have noticed that if your phone is receiving a text at the same moment you hit 'send' it will fail to send your text. Incoming texts will override your outgoing message. Just wait a second or two, and press the red exclaimation dot. Sometimes it may take a couple of tries but if you keep pressing that red dot, it will eventually go through. Hope that helps..
(No need for resets and battery pulls!)