PWM (motor speed control), the NewPing library, and the Servo library all need timers. Attaching one servo disables PWM on some pins (9 and 10 on a 328-based Arduino). Using the NewPing library disables PWM on some pins, too (I'm not sure which ones).
Which pins are your motors (via the shield) connected to?
From the Adafruit Motor Shield FAQ:-
"What pins are not used on the motor shield?
All 6 analog input pins are available. They can also be used as digital pins (pins #14 thru 19)
Digital pin 2, and 13 are not used.
Digital pin 11: DC Motor #1 / Stepper #1 (activation/speed control)
Digital pin 3: DC Motor #2 / Stepper #1 (activation/speed control)
Digital pin 5: DC Motor #3 / Stepper #2 (activation/speed control)
Digital pin 6: DC Motor #4 / Stepper #2 (activation/speed control)
These pins are in use only if the DC/Stepper noted is in use
Digital pin 4, 7, 8 and 12 are used to drive DC motors and Stepper motors via the 74HC595 serial-to-parallel latch
All these pins are in use if any DC motors or steppers are used
Digitals pin 9: Servo #1 control
Digital pin 10: Servo #2 control
These pins are used only if that particular servo is in use."