Why do ad blocks on Bidvertiser show the same ones?

I keep asking Bidvertiser support about this, and they keep giving me a form letter about geo-targeting of ads, but why are all my ad blocks from bidvertisers showing the exact same ads? From the advertising eligibility list, there are many advertisers who bid on my site, so why don’t successive advertisers show up on the 2nd and 3rd, etc. ad blocks? Anyone know? How can I make it so that different ads show up? Doesn’t bidvertiser know that that’s inefficient and results in a very low CTR?
It just seems so illogical. If I have 8 advertisers who bid on my ads, and two ad blocks on a page each capable of showing 4 ads, why are the same 4 ads shown on each one? Shouldn’t the subsequent ads be shown in the next block? This defies common sense.

Original post by Eurofile007