Is Assassination stack supposed to come from every attack?

[DarknessEternal]

Or is it a proc?

[Frostweaver]

It's a % chance based on the base recharge of the power. your quick attacks gave like a 50% chance, while your slower recharge attacks quickly stack up to 100%

[Croax]

And only the Primary Powers can give stacks. Pool or patron powers don't give AF stacks.