I am trying to control a robotic arm (https://www.amazon.com/OWI-OWI-535-Robotic-Arm-Edge/dp/B0017OFRCY/ref=sr_1_3/144-0256150-0926858?ie=UTF8&qid=1502310116&sr=8-3&keywords=robotic+arm+kit) using the arduino's digital outputs (I am using pins 12 & 13). If I wire up the wires to ground and 5V it goes one way, if I switch the wires, it goes the other way. However when I have the pins wired up to the digital pins, it does not work, but the onboard LED does blink.

The code:

int a = 12;
int b = 13;

void setup() {
  // put your setup code here, to run once:
  pinMode(a, OUTPUT);
  pinMode(b, OUTPUT);
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
  //up
  digitalWrite(a, LOW);
  digitalWrite(b, HIGH);
  digitalWrite(LED_BUILTIN, HIGH);
  delay(500);
  //down
  digitalWrite(b, LOW);
  digitalWrite(a, HIGH);
  digitalWrite(LED_BUILTIN, LOW);
  delay(500);
}