Problem with MDD10A + Arduinio UNO -- SOLVED!!!!

/// PROBLEM SOLVED. THE GROUND PIN WASN'T SITTING RIGHT. ONCE I CHANGED THE CONNECTOR FOR THE GROUND PIN, THINGS ARE WORKING FINE. //

Hi Folks --

// My set up //

Arduino Uno + MDD10A (Cytron - dual motor controller).
Four 12V 2.5A motors (12V, 118RPM 958.2oz-in HD Planetary Gearmotor - RobotShop)
Motors 1 and 2 (left side) are connected to channel 1 of MDD10A.
Motors 3 and 4 (right side) are connected to channel 2 of MDD10A.

The connections are as follows:
Dir 1 (direction left) --> Digital 7
PWM 1 (speed left) --> Digital 6
Dir 2 (direction right)-> Digital 4
PWM 2(speed right) -> Digital 3

The code is as follows ::

#define dir_left 7
#define pwm_left 6
#define dir_right 4
#define pwm_right 3

void setup() {
  // put your setup code here, to run once:  
  Serial.begin(9600);

  pinMode(dir_left, OUTPUT);
  pinMode(dir_right,OUTPUT);
  pinMode(pwm_left, OUTPUT);
  pinMode(pwm_right,OUTPUT);
  delay(5000);
}
int set_move(int speed) // set speed on each motor
{
  analogWrite(pwm_left, speed);
  analogWrite(pwm_right,speed);
  return 0;
}

void set_dir(int dir) // Set pins to high or low to decide direction
{
  if(dir == 0)
  {
    digitalWrite(dir_left,HIGH);
    digitalWrite(dir_right,LOW);
  }
  else if(dir == 1)
  {
    digitalWrite(dir_left, LOW);
    digitalWrite(dir_right,HIGH);
  }
}
void loop() {
    delay(3000);
    set_dir(0); // set direction to forward
    int speed = 100;
    set_move(speed);
    delay(3000);
    speed = 0;
    set_move(speed); // stop!
    delay(1000);
  
    set_dir(1);
    speed = 100;
    set_move(speed);
    delay(2000);
  
    speed = 0; 
    set_move(speed);
}

The Problem:

The right motors seem to respond to the commands well. They turn both forward and reverse at specified speeds.
The left motors don't. They turn okay in reverse, but do nothing in forward. When I press the test buttons in the motor driver (MDD10A), the motors work fine. Have tried changing the pins to no avail.

Has anyone else had a similar problem? Or is there something that I am doing wrong?

Thanks.

Warm regards,

What happens if you switch the motors around?

Thanks for

wildbill:
What happens if you switch the motors around?

Hi WildBill,
Thanks much for the response.
I switched the motors around - no change.
But, I did solve the issue. The ground pin wasn't sitting well. Once I set that up, it started working. I am marking the problem as solved.
Thanks again.