This is usually because one of the Objects selected has an Inner Join marked. 

Inner Joins make the fields available to specific object, required fields. This means if a user does not have that Object (Field) filled out in their profile then they will not appear in the list. Once you un-check Inner Join for that object the users who do not have that field populated will now display in the list.

Example: You have the User Address object marked with an Inner Join, the user does not have an address in their record but they do meet the criteria set forth in the Filter tab of the list. When you run the list this User will not display in the list.

Once we go back to Objects and un-check the Inner Join for User Address and run the list again, the user will display in the list with a blank fields for Address information.

User-added image