This works, just not on specific atlases. As stated above, other atlases in the same exact folder, called the same exact way, work perfectly fine.
foreach (Mech mech in mechDataScript.Mechs) {
GameObject newMech = NGUITools.AddChild (MechGrid, MechPrefab);
newMech.name = mech.Name + " " + mech.variant;
newMech.GetComponent<MechCard> ().mech = mech;
newMech.layer = 8;
newMech.transform.FindChild ("LabelMechName").GetComponent<UILabel> ().text = newMech.name;
mech.mechAtlas = Resources.Load ("Atlas/" + mech.Name) as GameObject;
Debug.Log (mech.Name);
if (mech.mechAtlas != null) {
newMech.transform.FindChild ("MechThumb").GetComponent<UISprite> ().atlas = mech.mechAtlas.GetComponent<UIAtlas> ();
newMech.transform.FindChild ("MechThumb").GetComponent<UISprite> ().spriteName = mech.Id.ToString ();
}