Some one said very often can’t get stack when using Calamity. I have tried to replicate this “bug” many times but failed to do so. I had isolated a zombie and hit it with Calamity with (weakened) HA multiple times. The stack increased when I pressed the HA key no matter the zombie was marked or not.
As, the stack is granted at the time you press the primary skill button, not when the arrow hit any enemy (marked or not), there is no correlation between the stack and the weapon or the skills you used.
Some players said it is actually caused by spawn multiple skills at the same time. I tend to believe this explanation more. A player had made a video clip posted on US forum before. I haven’t do intensive test on this.
As I said, I don’t believe there is any correlation between the “bug” and skills used including MotD. As I don’t use MotD in my build, I can’t confirm anything. If you have any find out, you are welcome to share here.