"Do you have hard water? If you have hard water, do you have a water softener? Hard water reduces the effectiveness of detergent substantially. Hard water can also contain a lot of dissolved iron, which can build up on fabrics and water-using appliances and fixtures. I’ve found that adding a cup of regular ammonia to the wash cycle gets towels cleaner and doesn’t harm fabrics like bleach does. It works well to break down grease and dirt and leaves towels soft and fluffy. I use plain ammonia or sometimes the lemon-scented kind. You can also pre-spray areas before you wash, ammonia is excellent on sweat stains. It doesn’t leave any lingering odor after washing."