Sounds like your friend took it to heart when your group called her immature. Mb she feels she can't be herself anymore and is annoyed w/you guys for "fooling around" / acting immature / being hypocrites (in her eyes).
Easy fix! Sounds like you gave the impression that you didn't want her to be immature at all anymore. You need to clarify what you/your group meant. It sounds like you enjoyed her being immature sometimes, but were mb annoyed with it under certain situations. Give examples of some of those situations to further clarify your point.
Finally, your friend cares about your group of friends a lot to have changed so much. Remember that!
I don't know if this is exactly it, but I've noticed that people seemed to have lost some sense of thinking or ability to analyze. Unless things are spelled out clearly, then they seem to miss out on subtleties. They might be one in the same- being too wrapped in things that they miss smaller observations.