If the shock has a hydraulic preload it may just need to be topped up with fluid and bled... depending on the design you may or may not be able to do this yourself. I would then compare actual preload adjustment travel to theoretical. I have an Ohlins shock that only gave 1-2mm of adjustment when I first bought it (used). After doing some research and bleeding the shock while still on the bike I was able to restore the adjustment range back to 8-9mm... however, with the adjustment knob turned all the way out there is still a small range where no "pressure" is felt, I imagine because the piston is pushing on the small amount of air left in the system.
