Unity bug with ray casting?

I'm having an issue with ray casting selectively working on some sprites that I am generating in a script. I haven't been able to find other people having an issue with this, so perhaps I'm simply initializing things incorrectly.

I created a sprite off screen, added a 2d box collider to it. I then need to create some amount clones at a later point, so I added this code in a script

ResourceItem = GameObject.Find("stone");
ResourceItem = Instantiate(temp) as GameObject; ResourceItem.transform.position = CalculateResourcePosition(); //just returns a vector3

At this point, everything appears fine, but when I click on the tiles, not all of them are clickable.

void OnMouseDown() {

  Debug.Log(string.Format("clicked {0} on {1}", this.name,GetMyTile().name));

}

For testing all of the tiles are in repeatable positions, so I've been able to determine that this isn't random (the same tiles fail to be clicked every time). It also depends on the position of the initial tile that I'm cloning (so changing its position will change which tiles fail). I've checked to make sure that the values for the collider and the layers are the same for the objects that do work and don't work. There's nothing else in the scenes to get in the way. Additionally, if pause execution, manually move a broken tile a bit, it'll work (so it seems like it's the relative position from my initial tile that's affecting the result of the ray cast).

Is this a bug with the ray casting algorithm or am I doing the initialization incorrectly? Thank you for any help.