To get the best from your Rockit Rocker use good quality Alkaline batteries. Zinc Carbon batteries are not powerful enough and will struggle to start the motor.
You can use re-chargeable batteries. NiMH re-chargeable batteries can last even longer than Alkaline and only cost pennies to charge.
Mount your Rockit Rocker as close to vertical as possible (with the button on the top). Battery life will be reduced if you fit the Rockit Rocker horizontally.
Try slower speeds, using the dial on the bottom. Sometimes the best rock can be achieved at a slower speed, it does not always need to be fast!