I think what is happening is that the girl feels vulnerable when she is with you. She is hiding the relationship from somebody/something…parents OR her own conscience. She might have a conservative upbringing and it is fairly recently she gave the thought of trying dating. When she sees you in person, her conscience self takes over instinctively and she tries to protect herself from you. She is afraid she might take things too far (like physical relationship) and this protective shell blocks any intruders like you. However, when she is online or over the phone, there is really not too many “mistakes” she can make; she is protected from any physical “miss-step” she could take; hence she is comfortable.

The solution? Create the atmosphere she will be comfortable with. If I were you, next time I approach her, I would act more like her friend than a guy she is in date with. Don’t display any innuendo and refrain from any physical/sexual move. Also for next few dates stop being overly romantic. Try to be her friend and crack jokes.

Try this out. If this doesn’t work there might be other complex issues which we need more information before passing our judgement.