When you log in to your AdEx account, you can see your account balance displayed in the upper left corner of the screen. That is your total account balance - disposable funds you can use for a campaign or withdraw + funds that are currently in a campaign.
We use micropayments, which means that payments happen per impression and not in bulk. As you can imagine, the price per single impression is sometimes rather small, so we credit these to your balance in chunks to make sure we save you Ethereum network fees.
Sometimes the money accumulated from a particular campaign is not sufficient to cover the network fees, so these don't show in the amount that is available for withdrawing. This money is still in your account, but our system "blocks" it until it grows to exceed the minimum network fee.
To see the amount available for withdrawal, simply go to the "Account" page.