The offerwall networks check the offer completions really carefully, as they are responsible to the advertisers for the quality of traffic.
That's why sometimes it may happen, that after you completed an offer and received a reward, the reward is then debited from your account.
If a user that received a chargeback has enough balance to cover it up, then the amount of chargeback will get subtracted from the balance.
If a user received a chargeback and there's not enough balance to cover it up, then the balance will go negative, and the account will be temporarily blocked. You'll need to contact us in this case in order to clear things up.