I am having a similar issue. I have 24 ads, in 6 slots. What I want is 6 ads to show, and each ad randomly chosen from the 4 ads assigned to the given slot. With "normal" I always get the same 6 ads, and with "random" I get as few as 3 and as many as 5 (rarely 6) ads, with more or less the desired behavior.
I tried setting up the default ad, but I don't want the default ad to show more than once, so that is not really the solution.
I am currently looking through the code, but I don't have a lot of experience with PHP, so it's slow going, especially since the code is not formatted for easy reading, and the comments are exceedingly sparse.
Site is http://amazopia.com