Dave's here to combat overdraft fees, but we also want to make sure that customers never fall into debt as a result of getting an advance.
We need to see proof that you have enough money in your account after you've taken care of all your bills. Basically, we want to see that you could pay Dave back with your next paycheck without going into debt. Because the last thing Dave wants to do is put you into debt unintentionally.
As a suggestion, try using the Dave budgeting tool. It can help you build up your account balance by telling you how much you can spend while still covering all your bills.