Why gray wood will not hold stain
The gray on a weathered board is a layer of UV-degraded fiber with mildew in it. Stain applied over it soaks into that dead layer rather than the sound wood beneath, so it sits patchy and lifts as the dead fiber lets go. The finish did not fail — it never got to the wood.
Clean first, at the right pressure
Softwood decking wants considerably less pressure than a driveway. Too much and the wand raises the grain and cuts stripes into the board — damage that stain makes more visible, not less. Even cleaning across the whole board is the goal.
Then wait for it to dry through
Not dry to the touch — dry through. Wood that is still wet inside will not take stain to depth, and sealing moisture into a board is a good way to get a cloudy finish. Typically a couple of days of decent weather, which is why cleaning and staining are usually two visits.
Choosing a finish
Broadly: transparent and semi-transparent finishes show the grain and need refreshing sooner; solid colors cover the grain, cover a lot of variation in the boards, and last longer on horizontal surfaces. Going darker later is easy. Going lighter, or coming off a solid color, means stripping first.
What actually wears it out
Sun and standing water, in that order, and both attack the floor long before the rails. A deck in full sun works far harder than a shaded one. Expect the horizontal boards to want attention while the rails still look new.