Edit -> Delete all by type -> Non Deformer History, если не поможет, то в гугле аналогичную проблему решали вот таким образом " I believe the problem was caused because duplicating faces from the body mesh caused its vertices to lose their smooth skin components (per the Component Editor) and thus Copy Skin Weights wouldn't recognize it as skinned even though by all means it was. I knew it was going to be something silly like that."