H.B.
this is going to sound crazy but trust me! i have fought with my tub for 5 years and FINALLY found the combo that works. The night before you want to scrub your tub, fill your tub with the hottest water possible and a capful of LAUNDRY DETERGENT! yup - and you MUST get detergent that has enzymes - any of the commercial ones do well (I like the Tide with the fancy lavender vanilla scent). Let it sit overnight - the detergent is going to get into all the dirty pores in your tubs and the enzymes are going to eat away the grime. The next day, rinse your tub really well. Then use Softscrub WITH BLEACH (use as directed) and rinse well again. I promise your tub will sparkle like a commercial!
good luck! and remember, don't use anything with ammonia (like scrubbing bubbles) when you're using anything with bleach.
H.