The soaking w/ diluted bleach worked for me. Maybe you diluted it too much? Try again, this time w/ stronger bleach. I did scrub it with Comet first.

I'm a housekeeper at an assisted living facility, and we have textured shower stalls in each apartment. I use the Mr. Clean Magic Erasers, and they do a great job but do require alot of elbow grease. The only thing on the shower floors the Erasers don't work on (for me) is the rust spots from the shower chairs. For those I use the S.O.S. Pads with lemon or orange in them.
So in your case I'd try the magic erasers first, and then if that doesn't work I'd get those SOS pads. I've found the generic magic erasers to be just as good as name brand, but the generic Brillo pads are not nearly as good as those SOS pads with the citric acid. You pay a little more, but they're well worth it!

Here's a method I found on the web that I'm thinking of trying; if you try this, let us know how it works !

Fiberglass Tub Floor Cleaner recipe
This is to be used only on the irregular bottom surface of the tub - not on the smooth surfaces.

3 cups baking soda
1 cup borax
Fine drywall sandpaper

Mix baking soda and borax together. Work the cleanser in with the drywall sandpaper and then rinse well.