could it not just be a matter of amount of resistance? In a push up, the person has a lot of weight to lift, usually much more than you would hand her in a dumbbell or barbell. Then there is the matter of shoulder girdle stability. Most chest exercises are done supine where the bench/floor provide at least some external stabilization.
Nancy made some good suggestions for alternatives to move in the direction of a push up.
As a matter of interest: why is it important to you that she does push ups?