The lvl 55 passive, Arcane Dynamo, grants stacks up to 5, allowing your next non-signature ability to deal an addition 75% dmg. Great. however, I've noticed several signature spells not giving stacks. Whether never rewarding stacks, or only giving some for the number of hits or damage I'm dealing.
At Level 56 I'm set up currently using:
1: Crystal Shell (Diamond Skin)
2: Stretch Time (Slow Time)
3: Energy Tap (Energy Armor)
4: Snowbound (Blizzard)
Lc: Piercing Orb (Shock Pulse)
Rc Obliteration (Arcane Orb)
Of the following Signature spells I've tried, Piercing orb seems to do it almost as it reads on the tooltip for AD. I figured it would be awesome if, let's say you used Living Lightning (Shock Pulse) to build up stacks extremely fast. And it worked! But then I tried Lightning Blast (Electrocute) as it's casting speed and hit rate seemed higher as well. I gained no stacks towards Arcane Dynamo, at all. I'm thinking this is a bug of some sort. Tried out a bunch of different spells, but only the lightning elemental ones seemed to have some trouble.
TL;DR: Yo, that passive seems bugged