There could be a a couple things here.
The name and the @ tag are not always the same. If I would try to send a message to @Tamara it wouldn’t get through to you because that’s your display name. I would need to address it to @justsewit because that’s your username. Could that be it?
The second thing it could be is something new–so it wouldn’t have been in place five months ago. Since that user registered and sent spam private messages to a number of forum members last week, we have put a new safety mechanism in place. Now you are only able to send private messages to people who you are friends with. This will help ensure that someone doesn’t register and send private messages to people they don’t know. So you’ll need to have an accepted friendship request from the person now before you’ll be able to send a private message.
Do either of those explanations help?