It's the way she lets Howard patronise her, I think. She needs to be more confident about her ability -- more like Lesley, who I miss -- and that's another thing, the fact that the writers dropped her because all they could think of to do with her was for her to taunt Sheldon and have casual sex. Because apparently intelligent women don't have lives. Sigh.
Of course, being me, I'd go for Lesley and Raj as thing. Lots of potential trying to solve his selective mutism through assorted unlikely experiments.