I've just started gunbreaker as a new class. I'm learning the opener rotation and am struggling. I feel like I must be missing something very simple, but something feels off

When I do the Keen Edge -> Brutal Shell -> Solid Barrel combo, I should be getting a cartridge at the end and a barrier in the middle (with brutal shell). However, I don't get the cartridge every time I perform the combo. Sometimes I don't get the barrier either. I also notice that the dotted yellow line denoting successive skills will sometimes pop up as a I perform the combo and sometimes it won't. Almost as if its registering that I have not met some type of prerequisite. Is GNBs main combo proc based? Is there some other interaction with the rest of the kit that I'm just not understanding?