Your tech levels are not what's limiting your breeding efforts, although breeding gets easier with higher levels of fertility. There are several reasons that villagers aren't interested in breeding: not enough housing, they don't "hit it off" with the mate you've chosen, or one or both of the villagers is too young.

You will experience a housing shortage four times in the game. The first three times can be resolved when your villagers build a new hut. The last time, however, indicates that you've reached your village's maximum population, since there are no more huts to build.