You were sending crap visitors, which is not what the advertisers want/pay for. Paint it however you want, it's still ripping them off. And here is most likely your problem. That's a lot of traffic in a short period. Analyse your sources and disable showing ads to any crap sources. There might be a lot of traffic from X, Y or Z country that your advertisers just aren't targeting or where there are very low conversions from. It doesn't matter which network you go with, none of them want crap traffic.