The reason may be that you have not written the post on time (within 2 days after its approval by the advertiser). After this period, the money earmarked for your payment is returned to the advertiser and will be debited from his account only when you perform the task. But by that time the advertiser can spend funds on other posts. In this case, when you send the link to the finished post, you'll be able to see a warning that the post can not be paid. You can refuse to perform such a task, or send a link to it for review. If the advertiser will replenish his balance, the money will be immediately reserved for you. But until he makes an additional deposit, your task will not be verified.