Royalties not being paid

My aunt and uncle have a well with rights that were with CNX, sold to Nobel Energy, then H.G. Energy, and now Diversified Energy (as of June 2019.) Their July check was received in September, then August and September checks were very late with the October check coming right before Christmas. They haven’t received any checks since. Diversified Energy is saying that they do not know how much they owe so they have been paying the same amount each month. My aunt has called Diversified multiple times. I will be calling tomorrow. Who should we contact about this? What are the steps to take when they stop paying or delay royalties for months?