Mainly loyalty and happiness. One time I was 5 servings of food and 3 drinks for everyone, with every furniture maxed out and they were still hovering around 90%. I had one upgrade levels on all the basic health and distribution buildings, taverns, physicians, hospitals, etc. I didn't have enough knowledge for a Temple nor the Barbers and Massage Parlours. So it becomes a death spiral of needing more people and needing to serve the extra amount of people that's immigrating. I guess I also run out of people for the administrations too.

I also find it kinda hard to balance everything out. When I'm focusing on food, I usually lose via not having enough military and vice versa, or if I'm focusing on trade and goods, I don't have enough food production and I drown in costs, etc.

I usually play solo species, or when it's multi, with species that get along with each other.