Improper loading: If the cups are placed upside down or the dishwasher is overcrowded, it may prevent the water and detergent from reaching all areas of the cup, leading to stains. Make sure that the cups are loaded properly and there's enough space between them for the water and detergent to circulate. Hard water: If your home has hard water, it can cause mineral buildup on the cups, leaving them stained even after washing. Consider using a dishwasher detergent that's formulated for hard water or adding a rinse aid to help prevent mineral buildup. Old or expired detergent: If you're using an old or expired dishwasher detergent, it may not be effective at removing stains. Make sure to use a fresh and effective dishwasher detergent. Age or quality of cups: Over time, cups may become more prone to staining due to their age or quality. If the cups are old or made of a porous material, they may be more difficult to clean and may become permanently stained.