There can be different reasons for this:
- The schedule is set when a new day begins. If you change it, it takes until the next day to become active.
- People have enough time, their other important needs are met, and important community tasks are available. This covers the need to start a family (provided it is unlocked, e.g. by meeting the age and level requirement).
- People have nothing better to do and all their needs are met, which covers the need for offspring and pretty much every other need (including higher-level needs, such as having fun or to relax).

As long as your villagers have the offspring need unlocked and are the right age, you are good to go.
It takes nine in-game months for a child to be born. May be everything is already in the making to get new villagers :D