Look at the bright side: getting ignored is a lot better than being avoided.
Now how do you realize you're being ignored?
- Do people interrupt you in conversations?
- Do they listen to you without actually taking into account what you said?
- Or do you have the impression that they talk to everyone else except you?
In the first two cases, well, I can give a few reasons even though I'm no professional.
First of all, aspies often use a very precise vocabulary, which most people aren't used to. I think those people are afraid to interact with you, because it makes them feel inferior. At least that's what some of my classmates used to say. It's not comforting at all, I know.
Besides that, have you noticed that many NTs love standing out? They speak loud, they make unneeded noises, they make big gestures. It's easy to get ignored if you're not as annoying as them. If that's the case, you'd better not worry about that kind of people. It's probably not worth it.
Finally, this could sound too obvious, but I'll say it for the sake of completeness: you will always be ignored in big groups (more than 3 people) unless you're giving a speech or something like that.
I'm largely ignored most of the time, but I've gotten used to it. I have a few acquaintances (not friends, but almost), and they never ignore me. Furthermore, I know that we say useful things for each other, so there's no reason to ignore anyone. My advice: find someone who appreciates what you have to say. Get in touch with people similar to you (not necessarily autistic, they could be similar in other ways). And don't try to stand out in big groups, it's not worth your time.