Are you using a brush to scrub the brick?

Yes, I have been using a white scrub brush.

Try Magic Eraser. I have stacked stone in a light color on the fireplace and it sooted up last winter. Nothing worked and the surface is not even making it hard to clean evenly. I bought two Mr. Clean magic erasers and it looks brand new.

I cleaned my sooty brick with Quick n Brite. It made a big difference but it was a big job and did make a giant mess but once it was done it looked so much better. I have the before and after photos somewhere! Check their website they have instructions for it. I know i used a stiff scrub brush and lots of rinsing. Just cover everything nearby because the scrubbing causes lots of 'over spray'. Its also non-toxic which is great.

ps. Once you try Quick n Brite you'll use it for EVERYTHING.