I just started in Exoclick and ran into this problem. I have sent in a ticket and am waiting for their response. I put 30+ campaigns in and the vast majority got rejected for "multiple similar campaigns". My second try was in a different country and I only lauched 2 campaigns, with different ad sizes, def. 2 different campaigns and got the same rejection again. Anyone had this before or know what's going on? 
Most ad networks hat copying campaigns. Leadbolt have same problem as well.
However, you can try to get several accounts, in which way you can get some nice traffic.
for exoclick they allow maximum 5 campaigns with similar banners targeting same placement, same country, even if the offer is different
Thanks for the replies - great tips
In case this happens to anyone else, I got a rep. from Exoclick on skype - very helpful - he had to reset my dashboard as everything I put in was now being rejected.
You can run a max of 10 ads per campaigns, max of 5 similar campaigns.
He said "You cannot add more campaigns with the same targeting. Please create a totally new campaign with different sizes, payment method, banners or sites or country targeting. Only changing the banner is not enough to get your campaign approved"