What the Hell Am I Doing Wrong with Spellslinger?!

Put skill in spellslinger. Assign that skill to a keybind, not a mouse click. Wait for charge. Use skill. Gg

Commenting to see answers, I have never been able to make spells linger work as I think the skill is intended to work

I'm also like ~99.9% sure that I understand how Spellslinger works, but just on the off chance I'm mistaken:

My understanding of Spellslinger is that it functions a little bit like CoC where you slot the spell(s) you want to use with it into support slot(s), but instead of Crit building energy, you build energy by casting spells, and rather than the slotted spell(s) firing automatically once you've built up enough energy, you need to manually trigger it by pressing the hotkey you've bound the Spellslinger skill to in order to cast it.

Hopefully that's correct, but please let me know if it isn't, and/or my flawed understanding might be contributing to the problem I'm having.

Are you sure you have enough mana/hp to use the slotted skills during combat? As far as I experienced, if you can't cast the socketed spells due to low resource, you still lose the stacks. Wouuld explain why you can cast it out of fight, because naturally your resource(s) are full.

You can also make spell slinger happen autonomously via num-lock on your keypad

Just use a num lock trick so you dont have to press it manually. Bind to num lock digit, get charges, turn on num lock, press binded digit and hold it while holding binded digit turn num lock off. Thats it.