Have you set the group and channel on the the SB600 and put it into remote mode...
(SIlly question but thought I would ask)
And on the commander mode have you set the b group up
does it work on group a? is the sensor on the 600 obscured? try using the sb900 as a commander and seeing if the 600 will work them
uh.... loads of things it could be :/
How fresh are the batteries in the 600?
when u switch the flash to wireless , of body, to recieve a command form a commander unit the red lights flash to let u know it in wireless mode.