Next time you buy lumber, look at the date stamp. Fresh wood is a terrible canvas.
Think of the stain as glue. A wet surface repels glue. It is that simple.
So, pour yourself a drink. Stare at your new deck. Resist the brush.
Your future self will thank you. And your deck will look like a million bucks instead of a science fair flop.
Go on. Wait. It is worth it. Trust the process, not the urge.