Armature spins within the motor.
Maybe you are having the same problem I once had on a well toasted motor. The magnets are attached to a ring of steel, that press fits on the axle hub. When hot enough, mine would spin. You could hear the motor spinning inside, but the wheel would not turn. Oddly, in my case it was nearly silent.
If you have any cash, you'd really enjoy the improvement if you upgraded to a brushless motor. When I dumped brushed hub, I really felt a huge increase in range and ability to climb a hill.
THE LIPO RULES. NEVER ABOVE 4.3V NEVER BELOW 2.7V DON'T PUNCTURE
Ideal charging /discharging range for Lipo, 3.65v minimum 4.1v maximum
See battery technology section, FAQ thread at the top of the page for lipo noob info.