L.A.
12 year olds it is time for us grown ups to start backing away from these relationships our children have. I know it is hard, but 12 year olds are usually in 6th grade, middle school. They are no longer elementary school babies. They are young ladies.
They need to begin leading the decisions about what THEY think they want to do about these things. Does she want to be invited? Does she want to know why she was not invited? Does she want to know these things bad enough to ask on her own? If not, There is her answer..
We never want to see our loved ones hurt, embarrassed, or overlooked. That includes by other peoples actions, but it also includes our actions.
IF your granddaughter brings up the subject listen to what she has to say, Then ask her, "well, what do you want to do about this?" "I can understand what you are saying, what are some ideas to find out?"
Also explain that friendships change at different times in our lives and really it ends up being for the better. We are all changing and growing in different ways.
Maybe your granddaughter is really not into what this other girl is now into (boys, makeup, Barbie dolls, video games, shopping). Maybe the other girl is taking your grand daughter for granted (they spend so much time together anyway). Maybe the guest list had to be made smaller or maybe there were other girls her mom wanted or felt needed invited because of other social reasons, or relatives..
I try to not think of the worst as to why something is happening, just in case it is a mistake or a circumstance I am not privy to.
You can set the tone in this for your Grand daughter. If you focus or harp on this, you are going to make her feel like there is something wrong when in reality, it is just another opportunity for Grand daughter to do something extra special with another of her friends or you..