Like men, womens muscles in their genitalia have varying strengths, meaning that her muscle strength may not be strong enough for you to feel her contract. This is not implying there is anything wrong with you or her for that matter. The strength, or lack thereof, seems to have little bearing on the womens enjoyment in climaxing any more than a man does if he can ejaculate halfway across the room or just dribbles. In simple terms if she says she is orgasming and you are in a long term relationship, why would she lie about it?