Apollo 125 oil seal
 

So my stepson has a new 125 and he was saying that the oil seal behind the sprocket keeps popping off on him. He was pretty upset I told him I would pick up the bike from his dad's house and fix it at our house but is there anything missing that allows the seal to work it's way out? I haven't seen the bike yet but would like to order any parts before I pick it up. Thanks in advance

Nothing holds the seal in ,check the trans vent it’s near the upper engine mount in the centre .