The first thing I can say would be to check the pinout/wiring of your transistor. You could also make sure you are providing enough base current for the current that is required for the motor at 2.4V.
You can check the spec for the motor at 2.4V for the current draw or just measure it with a DMM if you have one that can measure current. Then you can check the gain for the NPN transistor that you are using and check from there.