Shadow Conjuration not giving Shadow Absorption Stacks?

I'm new to the game and just picked Shadowmancer as one of my Wizard classes because deep down I'm incredibly edgy. My skill tooltips talk about stacking Shadow Absorption and interacting with it, but - while I only have Shadow Conjuration to build it - so far I haven't gotten any stacks as far as I could tell. None of the guides on the internet I saw talk about there being an extra condition to it, so I was wondering if it was just a bug, or maybe if the buff stacked but the buff bar didn't show an indicator, or if there was another caveat I should be aware of?