Virulence stack bug

Like the title says i can't build any stacks of virulence on any boss since the second part of the campaign.

I can easily get the 40 stacks within a second on packs but once i face a boss i can't build any.

I'm playing ball of lightning of orbiting linked to arcanist brand and even try without the brand but nothing works.

That's not a bug. The more stacks you get the more hits you need to sustain them especially against a single enemy. Against a single enemy you would need to generate about 60 hits a second. Things like The Golden Rule basically double the stacks you inflict and something like Lancing Steel of Spraying with return projectiles is much better for this than Ball Lightning. Against a pack of monsters you keep up the stacks because one Ball Lightning hits multiple enemies at the same time.