I believe you may have to build a new hut before you can increase the population. When you try to breed, is there a message across the bottom that says something about not being enough housing? I know that there is a post on here somewhere that lists the exact numbers, but I can't remember exactly where it is. Is there another set of the scaffolding to the left of the research table? If so, get your builders to complete that and it should allow you to produce more offspring.
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"Hope is the feeling you have that the feeling you have isn't permanent."
-Jean Kerr