I've sent several text messages to the number I text most often and haven't had replies to any of them which is very very unusual, i get the impression my texts are not being received. I've just googled my problem and there seems to be a few people who are having the smae problem, has this happened to anyone else???
Try texting someone else you can speak with to see if they receive it. The error could be that either yours aren't sending proper, theirs aren't recieving it, or she is ignoring you. Obviously the first two you would need to call o2.