Can't run two separated DC Motors

Question

I'm trying to build Bluetooth RC car, but I can't run only one DC motor.. only both of them. Everything works fine, the bluetooth, the app but when i try to go forward/backward/left/right nothing happens, only when i run both of them. I'm running the Arduino Nano with usb cable. The bluetooth connection is stable, The DC motors are working. I don't know what I'm doing wrong. Thanks!

const int AIA = 2;
const int AIB = 3;
const int BIA = 4;
const int BIB = 5;
byte vSpeed = 255;
int state;

void setup() {
pinMode(AIA, OUTPUT);
pinMode(AIB, OUTPUT);
pinMode(BIA, OUTPUT);
pinMode(BIB, OUTPUT);

// Initialize serial communication at 9600 bits per second:
    Serial.begin(9600);
}

void loop() {
  //Save income data to variable 'state'
    if(Serial.available() > 0){     
      state = Serial.read();  
    }


  /***********************Forward****************************/
  //If state is equal with letter 'F', car will go forward!
    if (state == 'F') {
      analogWrite(AIA, vSpeed);
      analogWrite(AIB, 0);
      analogWrite(BIA, 0);      
      analogWrite(BIB, 0); 
    }
  /**********************Forward Left************************/
  //If state is equal with letter 'G', car will go forward left
    else if (state == 'G') {
      analogWrite(AIA, vSpeed);
      analogWrite(AIB, 0);
      analogWrite(BIA, vSpeed);      
      analogWrite(BIB, 0); 
    }
  /**********************Forward Right************************/
  //If state is equal with letter 'I', car will go forward right
    else if (state == 'I') {
      analogWrite(AIA, vSpeed);
      analogWrite(AIB, 0);
      analogWrite(BIA, 0);      
      analogWrite(BIB, vSpeed); 
    }
  /***********************Backward****************************/
  //If state is equal with letter 'B', car will go backward
    else if (state == 'B') {
      analogWrite(AIA, 0);
      analogWrite(AIB, vSpeed);
    }
  /**********************Backward Left************************/
  //If state is equal with letter 'H', car will go backward left
    else if (state == 'H') {
      analogWrite(AIA, 0);
      analogWrite(AIB, vSpeed);
      analogWrite(BIA, vSpeed);      
      analogWrite(BIB, 0);
    }
  /**********************Backward Right************************/
  //If state is equal with letter 'J', car will go backward right
    else if (state == 'J') {
      analogWrite(AIA, 0);
      analogWrite(AIB, vSpeed);
      analogWrite(BIA, 0);      
      analogWrite(BIB, vSpeed);
    }
  /***************************Left*****************************/
  //If state is equal with letter 'L', wheels will turn left
    else if (state == 'L') {
      analogWrite(BIA, vSpeed);      
      analogWrite(BIB, 0); 
    }
  /***************************Right*****************************/
  //If state is equal with letter 'R', wheels will turn right
    else if (state == 'R') {
      analogWrite(BIA, 0);      
      analogWrite(BIB, vSpeed);   
    }
  /************************Stop*****************************/
  //If state is equal with letter 'S', stop the car
    else if (state == 'S'){
      analogWrite(AIA, 0);
      analogWrite(AIB, 0);
      analogWrite(BIA, 0);      
      analogWrite(BIB, 0);
    }
}

Answer 1

Connect the Nano ground to the driver board ground. Since they do not share a common ground, they can have odd behavior as their ground planes float.

You need only connect the grounds, the USB can still provide +5 to the Nano, and the +9 can still provide power to the driver. The only energy that will flow through the black wire connecting the grounds is the energy that flows between pins 2,3,4,5 and the driver's AIA, AIB, BIA, BIB, for the logic signals, which I'd bet is in the sub-uA range.