I have nearly the identical truck (97 K2500) and had this exact same issue. It took a little while to figure it out, but it is what you noticed; the airbag mount has rotated with respect to the axle tube. This causes a large part of the pressure in that bag to be pushing sideways and reduces its efficiency for lifting the load. Now, I took it back to the guys that installed it the first time, and they loosened up the u-bolts and rotated it back, and sent me home. Of course, it happened again soon after. I noticed that just by lifting up the truck again, it would rotate back on its own, even though the bolts were tight.
That's when I took a closer look, and the bottom line is that the kit for our trucks has a really stupid mounting plate, that basically is depending on the round axle tube to fit into a square opening...so just the corners are grabbing for friction...which is clearly not enough. My shop called to verify that they had the right kit...which they did. So what we did was weld an additional flat piece of metal that fills up the gap between the mounting plate, and the perch that sits on the axle tube....meaning that it cannot rotate any more. It's hard to explain without pictures, but the solution has worked just fine. PM me if you'd like more details, but we have the same kit on the same truck and had the same problem. Poor design in my book.