you know, this sounds unreasonable, but i seriously can't put my finger on it. it just looks off. it's still better than i can do, and very nice, but there's something about it...
tbh if I had to guess it would be the size of the glasses relative to the rest of the face would be the biggest thing. Also probably the mouth.
Reference:

I'm at a really frustrating point right now where I feel like all of my drawings are really good except for like, one minor thing that just sets it slightly apart from looking almost exactly like the reference.