FPS changing Rigidbody.velocity works on player but not on enemy

Hi, I'm making a FPS game and currently there is only the player and one monster (enemy). When I set WASD to change my player's velocity. Everything works fine. However, when I want to let the monster move by changing its Rigidbody.velocty, It doesn't work......

Basically, I did this to my player, with public RigidBody.

     Vector3 movDirection = Vector3.zero;
     if (Input.GetKey (KeyCode.W)) {movDirection.z += 1;}
     if (Input.GetKey (KeyCode.S)) {movDirection.z -= 1;}
     if (Input.GetKey (KeyCode.A)) {movDirection.x -= 1;}
     if (Input.GetKey (KeyCode.D)) {movDirection.x += 1;}
     movDirection = movDirection.normalized;
     Vector3 velocity = rigidBody.velocity;
     velocity.x = movDirection.x * MoveSpeed;
     velocity.z = movDirection.z * MoveSpeed;
     rigidBody.velocity = velocity;

And this to the moster object in Update().

     rigidBody.velocity = this.transform.forward * MoveSpeed;

No matter what number I set to MoveSpeed, I print out rigidBody.velocity and they were all tiny vectors and not equal to this.transform.forward * MoveSpeed.

I'm really confused since these two are basically the same. I understand that using Rigidbody.velocity may cause unrealistic behavior but that's not my concern here. Please help thanks.