I've got the GameCube and PC versions (using a gamepad) and noticed they both had this "issue." Much like the Turbo button (where after you hold it, you still have to wait a split-second before your character icon starts to rev up) the FSS button requires a half-second or so of "warm up" time... but while you're charging the ability, your player can only move! He can't shoot, he can't pass (because all the buttons are tied to that FSS ability).
For example, a steal leads to a fast break opportunity; I lob the ball down court to Andre Iguodala and I want to see a high-flying FSS dunk from the free-throw line. I hold down the FSS trigger just as I reach the paint, I press Shoot... and Andre continues to run?? Now if I didn't hold the FSS button and just did a regular dunk, that would have worked.
Another example: Rondo drives to the basket, but quickly kicks out to Allen. I hold down the FSS trigger and smash the Shoot button... and Allen's just standing there?? If I didn't hold the FSS button, Allen would have just tried a regular shot instantly.
I'm getting pestered outside, so Nelson throws a bullet to Howard under the basket. I hold down the FSS button and repeatedly jam on Dunk hoping to see Howard power through his defender... only he's still standing there, one second away from getting whistled for a turnover. If I didn't bother with using the FSS, Dwight would've... well, he would've tried a lay-up due to the AI/slider settings.
This isn't a stuck-key issue; with non-FSS players, I can hold the FSS-button down and continue to do actions. Is this what's supposed to happen? (While this may be a cause to abandon the FSS altogether, I'm a sucker for those instant-replays.)