You must be chased by Yang when entering the laboratory: this is mandatory to advance in this part of the story.
If you hide yourself (once or twice in the rooms you are describing), Yang will reappear: when getting the second key (i.e. key to the laboratory) and/or when entering the staircase map in the ground floor.
Though it's possible that there is a bug in some situations, and that Yang won't reappear any more. May be getting out and re-entering the castle could a solution in this case. May be (i.e. not tested) ...