I'm a subcontractor working on a facilities maintenance contract and the Prime is 5.5 months behind on payments and they have been paid by the Government. Unfortunately, I've learned that the government (Air Force) did not require a Miller Act Bond for the contract which is over $4Million over 4 years.
My first thought would be to double check that there was no bond. Typically, the Federal Acquistion Regulations require a bond for construction projects over $150,000. So, I would just double check that the GC isn't giving you incorrect information. Though, it could be that the prime contract is not technically a construction contract, so it's not subject to that requirement.
But, aside from bond claims, you could also have breach of contract claims. A construction lawyer can help you evaluate those claims and what you would be entitled to under your subcontract (e.g. attorneys' fees, interest). It's probably worth reaching out to a construction lawyer to evaluate your claims. Sometimes a payment demand and notice to file suit letter on a lawyer's letterhead is all that's needed.