Girls just don't listen to me!

I can talk to girls pretty well, but I notice a lot of times (even with girls I know), they won't even listen to me. I speak, and they just ignore. Now sometimes they may be talking to someone else, but a lot of times, she just isn't listening. Tips?

I seem like an innocent, nice guy, at least I (unintentionally) come off as one, which I expect is part of the problem. Any advice on correcting this? Girls just don't listen to me!