Bleach should only be used to remove mildew before applying a wood finish. If it lightens, it is probably mildew. Remove it with a solution of one part household bleach to two parts water, then rinse with water. If it doesn't lighten, it's probably just dirt, or wood extractives. Use Messmer's Part A or WD Liquid to clean the wood.
Decks are hard to keep looking new.
Wood traps dirt, dust, pollen and airborne pollutants, so you must remove them with a quality deck cleaner. We recommend cleaning your deck at least yearly. Twice a year is better. Don t reapply stain too frequently, this can cause blotchiness and shiny spots. Mix concentrates with water as per the label instructions, then apply with a garden sprayer or mop.