As a starting point, you would want to send them a formal, written request for payment per the provisions of the contract.  If they refuse the payment, then you may have to file a breach of contract suit.  You can do either on your own, but it would help if you have an entertainment law attorney help you with either step.  Often, if a company receives a demand letter from an attorney, it will motivate them to take a closer look at their obligations.  It also won't cost as much to hire an attorney just to send the first letter.  If they still refuse, you can then make the decision on whether or not to go forward with litigation.