Royalty Statement and OTC Gross Volume Differerences

I have mineral interests in OK, primarily in Hughes county. I receive monthly royalty checks from several Oil and Gas producers.

I have been comparing the gross volumes on my monthly royalty check statements with the gross volume production data reported to the Oklahoma Tax Commission (OTC) and the gross volumes from one of the producers is always significantly less that the gross volume production data reported to the OTC, no matter how far back in time I go. My other producers’ monthly royalty statements almost always match OTC production gross volumes exactly. The producer in question is not returning my calls or responding to my inquiries.

I don't know if I should be concerned about this and if so, should I seek legal council? Also, is there any other data source, besides the OTC gross volume production data, that has more accurate or timely information?