The sending and receving of PMs can also be determined by the person's forum name. If they have the same name as some one else; there's a high chance they might not get your messages.
There's a trick to this, I think Thierry pointed out. You had the member you want to send a message to onto your buddy list on the forums, from here, you go to send the PM; and in the send to section, you click "select members" this will bring up an option for you to search the member list. If you type in the person's forum name, and then select "from buddies only", it will bring up their in-game username, so you can be sure the message is sent to them and no one else.