I don't think you can do anything about it mate, if it's email I wouldn't know, if it's your phone then try switching it off and back on again as any unsent messages are sent automatically, failing that just go into your outbox and send again.
If you are talking about PM's from the site, outbox means it is waiting to be delivered to the person as and when they come on line next, once gone to them it then gets put into the sent box. Emails run on a similiar system.
What it all boils down to is when the person you send it to can be bothered to log on and read it. If it is still in your "OUTBOX" then they havn't read it yet for lots of reasons. 1 is that they don't like you or 2 they are too busy. If you get my jist.
Sent items means they have read it and now the ball is thier court to reply to you or not. Depends again if they like you or are too busy.