When you attempt to dive, it is completely random when he actually will. If you modify Pedstats.dat and change Dive Probability to 100, then the likelihood of you diving when hit R is certain.
The reason this is so, is due to a method of programming Rockstar chose, the Jock NPCs are taking a chance to dive every millisecond, due to the pedstats value, it doesn't happen every time it does. I hope i made sense there.