You sue the broker: if the broker is not paying amounts due under the terms of the commission or employment, etc. agreement(s) (whether written or unwritten/oral, since an unwritten agreement of this type is enforceable) between the broker and the agents, the broker is committing breach of contract. The agents can enforce the terms of those agreements in court and get a court judgment or order requiring the broker to pay as per the agreements. If there are multiple agents involved, it may make sense for them to jointly hire an attorney to sue the broker (so as to share or spread the cost).