You can sue for breach of written contract. Perhaps the threat of doing so will get cooperation. Send a demand letter first, by certified mail or fed-Ex toprove receipt, and give a short deadline to honor the agreement. Quote the contract's language that's not being honored. If the other party doesn't honor the agreement, sue in Small Claims court. Your rights and remedies depend on what's in that contract, so you may want to hire a lawyer to review it.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.