Not all aaa cells can deliver the amount of current that you need to power the motor. Measuring voltage tells you that the cells can out out 1.2 volts each (for example). But some low-cost cells cannot put out enough current (measured in AMPS). One way to see this is to measure the voltage on the battery terminals while the motor is running. A "good" pack will not drop all that much. A cheap pack will show less voltage when the motor is running.