Messages that you send will remain in the Outbox until the receiver picks them up. This is confusing and annoying, but the developers call it a 'feature.'
You will not be notified if someone sends you a PM. Without changing your default notification settings, the only way that you will you have received a PM is if you happen to log in to the forum and notice the little red envelope along the top.
Notification settings
To be notified by email when you receive a PM, or when a topic that you are following has replies, do the following:
Log in if you aren't already.
Click on your avatar and choose the User Control Panel option.
Top right hand side of page is an envelope and a bell.
Click on the envelope.
Click on message to read it. Might need to scroll.
If no message, maybe sender sent a blank page.