31 Operating not paying royalties or answering communications

My brothers and I inherited leases in Rio Blanco County in Colorado. The original division orders were from Koch, but during probate of the estate they sold to 31 Operating out of Rockwell TX. We have gotten several checks and statements, but sometimes it would take email and phone calls to get payment. Now we have gotten some statements, but no money actually came. They will not return phone calls or respond to email. What can I do to get the money they owe us?

Do you know if the wells are still producing? Have you checked with the Colorado Oil and Gas commission? Have you sent certified letters to the operator?

Thanks for responding. Yes, I filed a complaint with the commission and it came back two days later. They closed it saying all the documents were current and correct. They also sent forms for me to send by certified mail for payment. I have not done this yet as each well will need a separate form. I have upwards of 120 wells.