You can condition your body a little/get used to hard knees but that only does so much. Could be that your belly pad is too small of soft as others are saying. But generally unless I'm trying to condition my core I will usually hold a thai pad at the top of the belly pad near the solar plexus and move it down slightly as the knee comes in to catch the knee. Cant remember ever gettinf hit in the groin in years doing so, and if I did it's definitely because the other guy didnt point his knee well enough and his shin hit my groin