Synth pop not growing.

So I have a synthetic population, I produce 40 minerals and 39 power, I now have 4 worlds with a pop of 32 and now my synths won't grow, I already thought growing was stupid since I have already built them, but shouldn't minerals or power effect their growth? what does make them grow?

Solved: only 1 synth can be built at a time. -Mech9k, ticktockbent