2D platformer. Why the two enemies can pass through each other without collision?

THIS IS THE LINK OF THE PROJECT:
https://www.assetstore.unity3d.com/en/#!/content/11228

from the official demo.

Why the two enemies can pass through each other without collision

 

Make sure they both have colliders with IsTrigger unchecked. Make sure at least one has a rigidbody. Make sure they are both on layers that are defined to collide in the Physics2D collision matrix found in Edit -> Project Settings -> Physics2D.