The instructions on the bag suggest that a minimum level of 3 inches in the litter box should be maintained for maximum benefit.
I have also found that the older litter boxes that have been well used - it did have a tendency to stick. I find that I have to change my litter box about every year, and some folks also wash the litter boxes on a regular basis - which is also a good idea.
I've also found that the finish on the litter boxes wears with use and this too will cause the litter to stick to them.
And it also seems that some boxes have a better finish than others so I guess it all depends on the manufacturer of the box.
One other thing that I have found is that if a cat is passing larger amounts of urine - a health issue - it will have a tendency to stick as well.
I believe you are right - we should all take out stock in the litter companies. We could "potty train", but that's another story.