The reason most of those knees don't bring the other guy down is usually a lack of power. A fighter trains to absorb hits on less sensitive parts, which is a huge part in learning to defend against strikes. If you did not learn this during your MT training you should really ask for your money back.
The same goes for the groin, if you move and angle the body most of those knees will glance off against your hips. It takes only one clean hit, but that goes for the jaw as well so...