R.D.
What first comes to mind is odor. Does he smoke? Does he wear strong cologne? Does he have body odor? This may sound funny, but is his face symetrical? Is his voice loud or deep? Other than that, babies are usually an excellent judge of character. Maybe she just doesn't 'like' him. Her 'first impression' of him was not a good one. Or she may just be going through seperation anxiety around him since you don't see him very often. Maybe since she finally warmed up to him, next time will be better!