I'm definitely no expert on this kinda thing... I would suggest putting it all in a blender so it mixes thoroughly and whips.. and then throw it in the fridge. It usually sets decently for me within 5 minutes, literally that's about it and it's somewhat decent. you can leave it in for a couple hours or overnight to get it to set really good.
The only thing I can think of is either your fridge was not cold enough for some reason or more likely you used too much liquid vs. powder. That's gotta be it, try it with less liquid next time.
Good luck.
It sounds like you eat like I eat (convenience, bare minimum ingredients, eat cold food sometimes if u're in a rush), so I'm sure you'll find this a pretty decent dessert craving satisfyer if you get it right.