1 Does the planet have a leader?
If no then nothing you do will make it grow population.
2 Is the planet a Precursor World?
If yes then you have to build housing as Precursor Worlds are limited to one Pop till you do.
If 2 is no then you should be able to grow or build pops.
Synthetics need Promethium to grow but that does not sound like you problem.
If nothing above fits your game then there is something wrong with your game.
Try verifying files first then report if it still happens.
Let us know either way.