Why doesn't background fsck work ?

Hello!:

I tried to make my Nvidia video card work yesterday, and everytime
I launched the X system my computer hang up. So I had to 
make a hard reboot.
(I think I will fix the Xs problem this night at home)

But I've got another weird problem. :)

The fsck of my partitions is always made on the foregroud,
although I've heard about something like a delayed/background
file system checker.

Why is it always made on the foreground ?

-- 
JFRH