In some cases, such as with the Demographics module for Facebook Pages, there are certain criteria that have to be met in order for Facebook to return the data needed to populate the module.
In the case of the People Engaged tab of the Demographics module for Facebook Pages, the following criteria must be met:
100+ people must have liked your Page over the time period you've selected AND at least 100 of those people need to be sharing relevant data, e.g., their gender.
This means that, even if 100 people have liked your Page over the time period selected, if they're not allowing Facebook to share relevant information, such as their gender, this still means that there are not enough people for Facebook to return this engagement data to us.
The only thing we can advise in order for this tab to populate with relevant information is to select a longer period where it's more likely that the Page will have received 100+ likes from people allowing Facebook to share their gender information.