Doesn't matter how old it is. It can be a dud, or you might have verschnickered it. Which is what I think happened.

Check as advised the comm and brushes/springs, and take a skim if you have a lathe or find someone with one who can help you. If that does not rejuvenate its performance, you might have cooked the rotor.

Another possibility is that your motor is overheating in the race for some other reason (drag in the transmission, etc.).