I have the original KSG-12 (6+6+1). I keep slugs in one tube, 00-Buck in the other, and 00-Buck chambered.
Racking it is no different than any other pump action shotty. The fact that it's a bullpup creates a mental issue in how the user sees what needs to happen. The #1 mistake is trying to be gentle. You have to rack it like a man!!

As with any pump shotgun, if you don't give it enough force, it doesn't cycle completely. That includes BOTH the rearward and forward strokes. Worst case, you eject and waste an unfired round that leaves the magazine but doesn't make it into the breech.
Another issue with the user might be they are grabbing the forend too far back (toward themselves). That reduces leverage needed to get a nice, solid racking motion.
You may also tend to grab the forend too far forward. That's a major problem, because your hand is at risk of being in front of the muzzle. I added a flash hider/door breach muzzle device, mostly because it increases the length of the barrel another inch or more to help avoid reaching in front of the muzzle. The bullpup design is fantastic, but it also comes with the risk of blowing your fingers or hand off if you're not aware of where your forward hand is on the firearm.
I mounted a set of flip-up sights (no sights came with the shotgun), a Viridian green laser/light combo (super-small), a short vertical grip as far forward on the forgrip as possible, and a Kydex cheek rest.
The vertical grip allows me to choose if I need the pistol grip to rack it, or if I want to grab the forend itself. The grip acts in part as an index & safety feature, so my hand never extends too far down the barrel to the muzzle.
There was a design issue with the forend. The plastic rail on bottom can break if you have too much leverage and are puling at a backward-and-upward direction. I chose a short pistol grip to reduce the potential amount of leverage. I also bought a metal reinforcement kit that attaches to the lower rail. The grip attaches to the reinforcement. No fragile rail issues now that I can see.
I don't think anyone who practices a few times a year with the KSG -- or any other pump shotgun -- should have a problem properly racking it every time.
Be rough with your KSG. It
likes it!
