Why would people respond to invites with a "maybe"? Or even worse, not responding at all? I think they are trying to escape from certain things...
1. Escape from commitment.
Typical excuse: "I like to be more flexible. There are so many things happening. What if I find out later I can have better plans?"
Most of the invites are just throwing out an idea. Saying "yes" doesn't lead to rigidity. I would rather see a loud "yes" as a sign of passion. If you care about it, you will know it's a must-do, and you will plan other things around it. So I think those who say "maybe" just don't care.
2. Escape from responsibility.
Typical excuse: "I would rather not make promises than breaking them."
True, when you say "yes" to things, you become responsible. But if you have to miss out, an apology with sound reasons will always be accepted. It's just too hard for some people to be responsible and take things seriously.
3. Escape from being judged.
Typical excuse: "Well, it's cool to be mysterious. I don't want to be too predictable."
What's wrong with letting people know you are keen on certain things? A big problem these days is most of the people don't have a clear stance. They don't want to make a decision even on obvious choices. I think that's far from being cool. "I am who I am" sounds way cooler.
OK, MAYBE I am thinking too much... MAYBE, they just simply don't know what they want. I suggest to go the App Store and download "iChoose" for the iPhone. It's free.