How to deal with the situation that Stellaris has no population growth?

Population growth needs several conditions, which can be met before it can grow.

1 There is enough food. Without food, the population will not increase.

If there is enough happiness, too low happiness will not increase the population, then settling on those planets with too low livability may lead to too low happiness, resulting in zero population growth.

Slaves will not increase the population unless you choose slaves to reproduce in your policy.

If your policy allows free migration, even if the population of the planet with lower happiness increases, it will often jump to the planet with higher happiness. This is not to stop growing, but to keep people. . .

If you choose the attribute of "slow reproduction" when creating a race, then the reproduction of this race is quite slow. This is your own choice, and no one can blame it. . .