Sounds like your motor isn't getting much current. The 1K resistor in the base of the transistor is limiting the base current to 5 mA. Try something around 200 Ohms to bring the current up to 25 mA. That's still safe for the Arduino and should help make sure the transistor is fully ON.
See this article about using transistors as switches: http://www.electronics-tutorials.ws/transistor/tran_4.html