When you are attacking mobs, especially the RG, in greater rifts...

Where are you spamming your WoL?

Directly on the mob?

Or at some kind of angle that makes the pillar hit more than once per cast?

Drop your WoL directly next to the target you want to hit for ST. For groups just drop it smack in the middle.
Always make sure you're almost a full screen away to ensure Zei's is at full potential, also.