What if you threw a party without inviting them? See how they like it.

But really, it's immaturity on their part, and you won't be able to change them.

See if you can find other parties to go to, or throw your own. People can be weird like them, for no real reason.

If you can ask around and see what it is that they are mad about or just don't want to invite you for (better yet if you can talk to them without making it weird), then you'll know.