We filed a lien on a company that owes us for multiple items and I wanted to know if we can charge a monthly interest fee on the unpaid balance until they pay in full?
You likely can charge interest on the unpaid balance. Your contract may provide for interest on unpaid amounts. And if you can prove that payment was untimely under the Prompt Payment Statutes, Civil Code sections 8800 et seq., you may be able to recover 2% interest per month and attorneys fees incurred in recovering payment. You may also be able to recover interest under Civil Code section 3287.