Payment Disputes: Is It Possible to Get Royalties when Contract Did Not Include Them

My question involves independent contractors in the state of: nc

I illustrated a children's book that has been very popular in the past ten years since its publication. When I signed the contract I agreed to a flat fee of 3500 and nothing more. This book has since been made into paperback, hardback, and boardbook, in nine languages. I think the author is getting royalties, but I don't know for sure. The publisher has failed to send me copies of each translation as per the contract, but that is beside the point, other than to give just one example of the publisher's attitude toward me. I want to know how I can go about getting royalties, from the past sales, and for future sales. Is it possible to do this with legal help? I may be able to sue them for some sort of damages (concerning other issues not to be mentioned here) but I would rather just convince them to pay me a fair amount per sale, considering I put almost a year of work into it, my illustrations are what is selling the book, and it's one of their best sellers.

Thank you!

Payment Disputes: Is It Possible to Get Royalties when Contract Did Not Include Them