If you know you are riding solo, try it in the "Normal" preload position, you have been using IF it was fine for the 90% riding that you do (which was probably the way you were running the bike before you got the tool and wanted to ride two-up). Set it in the highest or next to highest setting (preload spring compressed) depending on how much load you are carrying. If it is any consolation...when riding two-up the suspension "sacks" down so what was a fairly tall solo bike actually sits lower with second rider and gear aboard, making bike easier to handle (make sure your passenger sits still and keep body aligned with the pilot, especially at low speeds.
If you have been scraping under carriage parts, determine what it is, as when riding two up, it will be easier to drag low lying parts. In the case of my sporty, I generally put it on the shock preload to the highest setting and rear tire to recommended 40 psi as stated in manual (my Metzler Marathons are rated to 50 psi if you believe what is written on the side wall) to accommodate heavy bumps or steep banked turns when two-up because my lowered bike will drag parts i.e. the jiffy stand tang on the left (I alleviated by cutting back the rubber bumper stop) and muffler bolts on the right (got shorter lower profile hardware). Also helps learn how to "counter-lean" or "hang off" on the inside to stand the bike more vertical to improve cornering clearance.
