Still can't get stains out of mattress - what am i doing wrong?

Probably too much water. If you soak a mattress the stain wicks deeper and you get a brown ring as it dries. What's the stain? For sweat or the general yellowing, a paste of hydrogen peroxide, baking soda and a drop of dish soap, spread thin, left to dry and then vacuumed off, works better than scrubbing. For pee or anything organic use an enzyme cleaner and let it sit. Test the peroxide on a corner first, and get a fan on it after so it dries fast. Old yellow stains that have been there for years honestly might only fade, not vanish.