What might happen to populations that cannot get enough resources?

It is clear to all of us that the planet is not expanding. There is only so much space on Earth, not to mention only so many resources that do have a limit. It is not the number of people on the planet that is the issue, but the number of consumers and the scale and nature of their consumption of the resources. So the answer to your question is that the people that cannot get enough resources will eventually die off.

They will eventually start to die and become extinct.