I'm using a 27" iMac with Mavericks. About 2 weeks ago, Messages stop creating notifications whenever I received a message from someone. I used to get a notification in the upper right corner of the screen if Messages was in the background, but now I don't get anything. As a result, when people send me IM messages while Messages is in the background, I don't see them until later when I see the Messages window.
I didn't change any settings or anything like that. They just stopped showing up.
I don't see anything in the Messages Preferences about turning Notifications on/off either.