Stains on the bottom of a tub can be very hard to remove. The approach you take to get rid of them really depends on what kind of stain it is.
If your water has high iron content, you may be experiencing rust stains. These are especially hard, because common household cleaners just don’t cut it. If cleaners formulated for calcium, lime, and rust won’t work; you may need to order some oxalic acid. It comes in a powder form, and you make a paste by adding water. Scrub the surface, allow it to sit 20-30 minutes, then rinse clean.
For other tough stains, try combining equal parts of cream of tartar and baking soda with enough lemon juice to make a paste. Use a soft cloth to rub the mixture into the stain. And again wait about 30 minutes before rinsing clean.