I used to find that even with weekly washes, my dazzling white bed sheets quickly faded to gray. Months of detergent build-up to remove yellow bedding stains left a film on my bedding, a problem worsened in hard water areas, as mineral deposits add another layer of dullness.
My first instinct was bleach and a hot wash. However, while chlorine bleach can strip away soap residue, chlorine reacts with mineral build-up to lock in hard-water film. Despite the heavy washing, my gray sheets were even duller.