Adjustment layer not working (has no effect)

I'm having a problem with adjustment layers... Here's what works :

- I can add an effect (Blur) to a normal layer, and I see what it does in the Comp preview window and when I render the movie.

What doesn't work :

- I add an adjustment layer above everything in the timeline, add an effect (Blur) to this layer, but then nothing happens neither in the Comp preview window or the final render !!

I've tried moving layers around, enabling anything related to effects, and still haven't managed to get this to work.

After effects 6.5 (old I know... but it works just like CS3 & others, and just takes 150MB of disk space !!)

Win XP

Any ideas ?

Thanks !!