No, the lienholder is not responsible, since the lienholder is not the one who entered into an agreement (whether written or oral/verbal) for you to do the work. And the liens from the financing or leasing do have priority over the debt to you and so the lienholders are entitled to the cars if they were not paid. You can sue the customers (such as in small claims court) for the money; in the future, you may wish to take a page from attorneys, who typically make sure to get a retainer, and at least get partial payment or a deposit upfront to protect yourself.