There is a few ways the mobs loads. sometimes you wont get all the mobs you need. for this all you need to do is kill Thor. he is tough. To get to Thor you will need to unlock loki. Vali has his key. if you fight vali for a while till you almost die, he will stop the fight and say your tougher than he thought. after you heal up he is easy to kill. get the key from his corpse and unlock the fetters at loki. after you unlock the fetters with loki he will give you a ring to give to Sif. (he dont always load) if Sif loads he will give you access to Thor. you can hunt him.