xx_satanic_mechanic_xx says:

This is a common misconception when people trade in cars. You are responsible for the payments on that trade until the dealer is funded on the new car purchase and pays off your old one. You are still on the hook for that old loan, so make your payment. when the deal is finalized at the dealer, and they have been payed by the lender for your new car, they will pay off the old one.

If something goes wrong and this deal does not go through, the car will come back to you with the old loan in place – and paymetns 30+ days late. So keep em up.

This is your credit. You are the only one with a vested interest in protecting it. Send in the payment and then contact the dealership to find out what the holdup is. But again, make no mistake, you are still responsible for that old loan until it is paid off.