Charge more. Your rates need to have credit risk built into them. Unless I miss my guess, they don't, and they're at a discount to market to boot.

How are you sourcing "They're going under"? If it is true, you won't be able to collect -- if the bank account is bare then it is bare, and there is no reason to expect the founders to backstop this invoice for you.

If you're simply being fobbed off, then the usual escalation options are a) restate demand for immediate payment via certified mail and b) lawyer sends them a nastygram.

But either way, charge more.