Static BL not spawning /w Arcanist Brand on certain Enemies?

First time ever using brands, but I can't find any info on this.

Using Static Ball lighting with poison on arcanist brand, which works fine except for some bosses or random enemies it doesn't seem to spawn, I can see the brands are activating but no ball appears.

Mainly notice it on some unique enemies (wildwood bosses etc) and wasn't sure if this was an intended mechanic.

Definitely have enough mana, I can see the brand activating.

First noticed it on Dusk, Harbinger of Lunaris (Act 8) and same on Dawn, Harbinger of Solaris and it's happened a few times since then.

Pretty sure in all cases whisps are active (or I'm in the wildwood), but can't be sure.

Noticed another user has issues with Static Ball Lighting and traps as well, so thought this might be related?