The clumping tendency is definitely affected by changes in humidity.... lower RH = flows freely (sometimes associated with static problems) and higher RH = the start of clumping. The type of coffee bean, the degree of roast (oily beans) and the grind fineness also affect the tendency to clumping so in the end, it can be due to a combination of all of these factors as to why you experience it with your Rocky D/L.
I really only experience it with my Rocky when using Oily Beans, as the grind setting I use doesnt change all that much, maybe +/- 2-3 steps max. When clumping does occur, its never really worried me all that much though as after tapping on the bench a couple of times and then tamping down, always get a great brew afterwards and thats the main thing isnt it?
Maybe just try some different beans or better yet, roast some of your own ;D and see how that goes. I dont think it is really a big deal in the end,