Attribution Window vs Learning Phase

Attribution Window is when Facebook attributes certain actions to your ad (such as a purchase of a product) if they happened within 1, 7, or 28 days. 

The Learning Phase is when a campaign has just started and Facebook does not have all the data necessary to deliver it in a stable way. In order to get that data, ads have to be shown to different types of people to learn who are most likely to get you optimisation events. Your ad set needs to get about 50 optimisation events in a week to complete the learning phase.

If you don't get enough events in a week, that may be because your attribution window is too narrow. The solution would be to broaden the attribution window so that more results are obtained.