Low Fill Rate.
One thing I've noticed is "Fill Rate". I do know you want this as high as possible -- I get that much.
According to my data, I have a fill rate as low as 30% on some ads. And that is between my main network and the backup. I've also noticed that sometimes (quite a few I think) ads don't even show up at all.
Is there something I can do to correct this or is the problem with my networks?