Campaign manager has rights to choose anyone to be a part of their campaign but most managers will tell you the reason why you didn't get accepted but don't get butthurt and start hating them which will reduce the chances of that manager's future campaigns.

If you got too many posts applying for campaigns then the manager might have get a feel like you are here only for making money so don't apply on too many campaigns in a row, apply on one or two and wait for the results but don't stop posting once you applied for a campaign.

Every one deserves chances and even those listed on blacklists by managers will get chances after the cooling time period ends. You can read lot of rank up stories from people who labeled as shit posters earlier.