We're a subcontractor that did work on a project for a Native American tribe. The General Contractor is no longer responding to our emails or calls. There is still money owed to us which we have billed through signed change orders and a signed contract. We don't necessarily want to take action against the tribe as we know how complicated that would be. What we do want is some document directed toward the GC which we can also send to the tribe to get everyone's attention. What actions can we take from here that will get the GC to respond to us?