Mana Remnants seemingly not triggering despite being active and many lighting/shock effects - help?

My wife is using a Sorcerer Disciple of Varashta with many Plant and Lightning skills

She’s following a build guide but one of the key parts of the guide seems to indicate that by late game some of her mana problems (which, to be fair, she doesn’t struggle too much with) can be solved by Mana Remnants

She uses Thunder Storm to “drench” enemies and the a few different lighting spells to try “shock” them (with a bunch if supports seemingly geared towards this end)

The issue: she’s had this setup for a while and I’ve maybe only once witnessed it trigger, and she seems to think it NEVER triggers… ?

What are we missing here?