Cannot Add Voucher to Batch Once All are Approved

We're trying to add additional vouchers to a batch, but it is not allowing us to do so. Why?

If your organization utilizes the voucher approval process and all vouchers in the batch are approved, the system will not allow you to add additional vouchers. You will either have to un-approve one of the vouchers so that you can add additional vouchers, or start a new batch.