It may have meant that the whole thing was just becoming too tense for her. Perhaps she just couldn't deal with the anxiety of the french kiss. Maybe she was standing there thinking to herself, does he think I am a bad kisser? She then may have decided to just stick to closed mouth kissing.

I think she might have a good idea for the present moment because she is taking the pressure off the situation. What you may want to try is simply relaxing, realizing that she is as nervous or as embarrassed as you but that you both want the same thing--to say good night to each other and to have a nice kiss. There is no reason you shouldn't get to this point.

Remember to relax and remember that practice makes perfect and here you have a girl who seems to be at the same level as you so you can both learn from each other. Maybe the next time you are alone {preferably not at the end of the date} ask her if she wants to just try to french kiss again just for fun to see what will happen. If the situation is comfortable and there seems to be no pressure, you will probably both be able to relax and concentrate on the kiss itself (feeling more free to experiment) and not on whether the other person is judging you.