Arduino uno + cnc shield v3 + Stepper Motor Driver A4988 + AccelStepper? I successfully run the AccelStepper examples on an Arduino Mega with a RAMPS 1.4 shield, two A4988 stepper motor drivers and two 17HS16-0284S stepper motors. 

I used 3 jumpers on the RAMPS to set the microstep resolution to 1/16.

I used this code inspired by the Polargraph Server for Polargraph:

#include &lt;AccelStepper.h&gt;

/* 
// for the Arduino Uno + CNC shield V3

#define MOTOR_A_ENABLE_PIN 8
#define MOTOR_A_STEP_PIN 2
#define MOTOR_A_DIR_PIN 5

#define MOTOR_B_ENABLE_PIN 8
#define MOTOR_B_STEP_PIN 3
#define MOTOR_B_DIR_PIN 6
*/

// for the Arduino Mega + RAMPS 1.4
#define MOTOR_A_ENABLE_PIN 30
#define MOTOR_A_STEP_PIN 36
#define MOTOR_A_DIR_PIN 34

#define MOTOR_B_ENABLE_PIN 56
#define MOTOR_B_STEP_PIN 60
#define MOTOR_B_DIR_PIN 61

AccelStepper motorA(1, MOTOR_A_STEP_PIN, MOTOR_A_DIR_PIN); 
AccelStepper motorB(1, MOTOR_B_STEP_PIN, MOTOR_B_DIR_PIN); 

void setup()
{
  motorA.setEnablePin(MOTOR_A_ENABLE_PIN);
  motorA.setPinsInverted(false, false, true);
  motorB.setEnablePin(MOTOR_B_ENABLE_PIN);
  motorB.setPinsInverted(false, false, true);

  motorA.setAcceleration(100);  
  motorA.move(1000);
  //motorA.setMaxSpeed(100);
  //motorA.setSpeed(100);

  motorB.setAcceleration(100);  
  motorB.move(1000);
  //motorB.setMaxSpeed(100);
  //motorB.setSpeed(100);

  motorA.enableOutputs();
  motorB.enableOutputs();
}

void loop()
{

  motorA.run();
  motorB.run();

}


I tried to use the same code with the same stepper motor drivers and motors on a Arduino Uno and a CNC shield v3 without success (I also used the 3 jumpers to set the microstep resolution to 1/16). The motors do not work and jitter.

What is wrong with my code?

Bonus questions:
I have taken this setup and code from the Polargraph server but I didn't get how I'm suppose to know how to use setEnablePin() and setPinsInverted() methods. In fact, I have troubles finding documentation about the CNC shield V3 and the A4988 drivers (I only found this page) to work with AccelStepper.
Manually set the ENABLE pins as OUTPUTs, i.e. add in the setup() function:

pinMode(MOTOR_A_ENABLE_PIN, OUTPUT);
pinMode(MOTOR_B_ENABLE_PIN, OUTPUT);