Is there a statute of limitations to make a claim if operator did not escheat monies to State of Texas

I've recently learned that the operator was paid directly for the casinghead gas produced from a well on land my mother held mineral interests. He was receiving payments as far back as 1996. I inherited the mineral interests when my mother passed in 2001 and have been able to collect the monies from the oil companies either directly through new division order being issued or through the State of Texas Unclaimed Property Fund. The operator however never escheated her share of the casinghead gas monies to the State. I have made a claim to the operator for all monies (1996 forward) but they are saying because of limitations they only have to go back 4 years when calculating the monies owed. If this is correct, then is there some other avenue I can pursue to get the monies that the operator should have escheated to the State.

Have you ran this by the Corporation Commission ? They swing a big stick !

John