It depends on the stain and the bathtub composition. My go to would be Mr. Clean Magic Eraser. You might need to add something to that like vinegar, baking soda, or hydrogen peroxide.
You might try contacting a Norwex representative if the eraser doesn't work.
If it's not marble, I would also recommend a no-scrub product "wet & forget shower".
try Coke a cola
what color is the stain and where is it?
Hi. Typically tubs don't stain unless it's rust or some other mineral. You can actually try a toilet bowl gel cleaner for rust and hard water stains. If that does not remove the stains, if the tub is old the porcelain may have worn thin and your are seeing the metal. If that's the case, consider re-doing the tub with an epoxy paint. Happy DIY'ing.
I have no answer, only another question. I have a hair dye stain on my plastic tub. Now what do I do?