OnTriggerEnter2D isn't called consistently

Hey guys,

I need your help! I'm two days with this problem, I don't know what to do. =/

I have two objects: Player and Enemy (Spawn). When I press play for the first time, the Animation works very well at the collision.. But when a press play for the second time, sometimes works, sometimes not.

Please see it:

(Player)
void OnTriggerEnter2D(Collider2D collider)
{
Debug.Log("Before if");
if (collider.gameObject.tag == "Enemy")
{
Debug.Log("After if");
_animator.SetBool("hit", true);

StartCoroutine(WaitSeconds());
}
}

IEnumerator WaitSeconds()
{
Debug.Log("IEnumerator");
yield return new WaitForSeconds(0.2f);

_animator.SetBool("hit", false);
}

As I was saying, when it dosn't work, OnTriggerEnter2D isn't called. I had a suspicion, but I don't think is it. I have OnTriggerEnter2D in Player and Enemy's Class, but both do different tasks. And I wanted to keep a low coupling, Therefore, isolated classes.

In the both classes, I checked "Is Trigger" at the Circle Collider 2D, and in Player I put the Rigidbody 2D.

If necessary, I can show some prints, or the code or a video! =]

Please... Help me! =(