The reason you're unable to select certain custom audiences is because you don't have the minimum number of people required in them. Just to let you know, Facebook's minimum size for custom audiences is currently 20 people.
In order to add more people to the custom audience all you need to do is the following:
1. Navigate to the 'Set targeting' tab.
2. Click on 'Manage custom audiences' in the top right hand corner.
Alternatively you can update the audience in Facebook directly for use in Qwaya.