Clean it(Including the mag. Don't ever lose the mags either. Expensive to replace these days. $29.80US from Gunparts.) and change ammo. Try a box of as many brands as you can to find the ammo that both shoots well and cycles the action. The price of said ammo means nothing either.
Cooey 64's are fairly forgiving about the ammo they'll cycle with, though. Not so much about accuracy. Mine like Remington Yellow Jackets and most regular high velocity ammo. Doesn't shoot anything particuarly well.