Stuck in story mod possible bug.
If I recall correctly at one point the same happened to me. It was because a single enemy that I was fighting managed to disappear without being killed. Eventually he wandered back into the middle of the group but yea just try resetting it and make sure to kill everything in the event battles.
yeah did that now , 1 mob disapeared and seemed like never came back . Reseted the instance and it worked ok now.