How to fix newbies deck stain mistakes without sanding the whole deck
Well, let's see what the instructions say.  


  Apply using an exterior paint pad, brush, roller, dipping or compression driven sprayer. If over applied, remove excess before it dries by redistributing to dry areas or wiping off...


Make sure you did that. Honestly when working with a coating for the first time, I cut my teeth with brush/roller/tip-out or whatever is the "normal" way to apply the product. I only upgrade to more "efficient" methods after I have Popeye arms from doing it manually.  A lot of people leap straight to the most tech-heavy or "efficient" method, only to get snagged up in unfamiliarity with the product or tool.  Case in point. 


  Only one coat is recommended. A second coat can be added if more color is desired. 


This is paint company talk for "Two coats required" :) 


  Allow the first coat to dry approximately 2 hours. Second coat must be applied within 4 hours of initial application.


Every product has "recoat times", when the coating is stable enough to take a second coat, but still soft enough for the next coat to chemically bond.  Here are the rules for this coating. 

When you miss your recoat times, you must now "scuff-sand" the layer.  You're not trying to tear it down; you're just trying to lift the gloss off the surface, and give it microscopic mountains, to make the surface rough enough for the next coat to *mechanically bond.  

This isn't a full-on sand-down.  You can get it done with green Scotchbrite pads or steel wool, or a wirebrush. 

Then I would try a second coat and if necessary a third; so plan your day so that you can hit that 2-4 hour recoat window. 


  Keep the leading edge wet and distribute waterproofer evenly. This will help avoid lap marks and keep the color uniform.


And pay very close attention to this. When you don't keep a wet edge, your overlap becomes a second coat, deepening the color.   This is yet another reason not to be "exploring" a new combination of product and application equipment.  I would expect the normal application method to be roller/tray; stick with that.   Google "tip out brush" for a way to make rolling even more uniform.