I am posting this in case anyone else has this problem and comes across this via googling. I started having a problem with my messages sending around Nov. 20th and I thought it had to do with my data being throttled but when I was no longer throttled and the problem continued I started playing around with the settings. Some of my imessages would go through while others wouldn't (but it was the same in that, it always went through to the same people and always failed to the same people.)
What I did to fix it was, settings-> Messages-> turn off imessages -> reboot-> turn on imessages ->sign in to imessages -> sent test text to someone that it kept failing to sent to.
(I am currently on ios 8.1, 128GB iP6+. I suppect I have the model with the bad memory.)
What I did to fix it was, settings-> Messages-> turn off imessages -> reboot-> turn on imessages ->sign in to imessages -> sent test text to someone that it kept failing to sent to.
(I am currently on ios 8.1, 128GB iP6+. I suppect I have the model with the bad memory.)