When team is on a run, players miss free dunks, why?

Every time when a team is on a run, players tend to miss constantly dunks on fast breaks, why??
And this happens to good dunkers like Garnett and Mcgrady.

try turning up the dunk percentage slider.

It's the catchup logic in effect, though personally I think it's a little better than last year. If CPU Assistance is enabled then you'll see it happen fairly frequently too.

But it's ridiculous for Garnett to miss a dunk under the basket when no one around him time after time!!

yes which is why most people dont play with cpu assist on....

So cpu assist works for my team and computer team as well?

But it's ridiculous for Garnett to miss a dunk under the basket when no one around him time after time!!

Indeed it is, but that's what the CPU Assist option (and to a lesser extent, the built in comeback logic) does. It's not a perfect solution to the issue of keeping the game challenging, though at least it works for both the CPU and the user. The CPU's comeback logic is of no help to the user.

I'd recommend playing with CPU Assist off. Playing on Superstar difficulty should be enough of a challenge with the CPU's comeback logic and an extra challenge can be achieved by tweaking the sliders to give the CPU an unfair advantage.