Some people believe that you get stuck into an unfinished group. So take for example a Syrcus tower party you need 1 tank that has english clicked. But the tank ques up with a healer friend and starts a whole new group. So new dps and healers are filling up that group while your group doesn't fill up.

I understand that doesn't truly explain what happens. But SE refuses to comment about the system they use. And all we can do is use observation. As I am sure you have seen no oe lets que get to 30 mins. And in fact if I know a new match has popped I reque. I never get any penalty and on a busy night I almost always get into that match. Its also why in linkshell if I know people are queing I say pop when I go in. Because I have actually had matches where I knew 3 people were in que and it never filled.