I often find people who seemed to have auto picked a match just sitting there stuck in an empty match.
What seems to happen is that people will set up a team match and take only friends for their team. A couple of people will auto fill into the opposing team but they never fill up their friends team and eventually leave.
The people who auto filled though can't see that unles they're actively looking at the duel page. If they joined up then walked away from the duel masters they never see that the other team has left and will not be coming back. All they can see is that their team is full and is waiting on the other team to show up.
Meanwhile others are making matches which they CAN join but they can never see it.
Possible fix is change the display which shows yuo your team to show you both yours and the other team so you can see how they are filling up, and rather it's a friends only match you've ended up joining.
just recently i got rid of my membership so i will be having the same problem when i go to the arena, i think that you should make to seperate sections, one for friends and one for people who battle randomly, i am sure it would cause less complaints :)