Try hydrogen peroxide,I know it sounds dodgy and in concentrated quantities it is but if you use 3-10% concentrate then it should be fine and it's great to watch it all bubble up,don't worry it's absolutely fine infact hydrogen peroxide or its chemical name h2o2 is just water with and extra oxygen molecule added to the mix,it will cause no damage as long as you stick to the suggested concentrates.It kills all bacteria and will definitely loosen up those hard to get rid of stains.