Collider and raycast hit is not working properly

I’m making a bird shooting game … when I hit a bird(which has a collider) with a bullet(which has a collider and a rigidbody) … Sometime it hits and sometime it passes right through it. But when I enhance the collider of the bird, it always hits. Just so you know, bird collider is still big enough to take a hit but sometimes it do and sometimes it don’t.

Start()
{
    ForceShoot = 500f;
    initialVelocity = this.transform.forward * ForceShoot;

    this.rigidbody.velocity = (initialVelocity);
}

FixedUpdate() {
    if (rigidbody) {
       this.transform.forward = rigidbody.velocity.normalized;
  }
}

PS: I have check hit with both techniques, RaycatsHit and OnCollisionEnter

maybe this is the problem: