RigidBody2D Player gets stuck...

Hello
I am creating a 2D game and I am not using sprites I am using actual 3D objects.
I am using Box Collider 2D by the way.
I added a cube and I added this script to it.

Code (CSharp):

1.  
2.     public float speed = 2f;
3.  
4.     void Update ()
5.     {
6.        rigidbody2D.velocity = new Vector2(speed, rigidbody2D.velocity.y);
7.     }
8.  

Everything works perfectly but the problem is that the cube sometimes just stops suddenly.
It's like that cubes box collider is a little to high, the think is if I restart the game it might stop somewhere else.
It happened before and I didn't know what was the problem.
If anyone can help it would be great, thank you.

 

try using a circle collider instead...they don't get stuck

 

Ok I will give it a shot, thank you.