For me, blocking layups and dunks are absolutely impossible or determined completely by luck. I've turned my lay-up blocking ability up to 100, yet still the only blocks I seem to get are from jumping randomly as the guy runs towards me, or a block from behind (which usually happens when I don't have the shotblocker selected).
I just had Earl Boykins run up to Marion on a 1-on-1 break and make a layup in Marion's face as he jumps up to block the shot. That's just pathetic.