I noticed on your image, the attempts from drop down says enrolled users--try changing it to all users? Maybe it isn't recognizing your users as enrolled? You might check your enrolled users. I know sometimes on my site people get enrolled with no role (I don't think that usually lets them participate, though, unless you do things different than I do) and that throws things off.
Also, if they are allowed to preview, I'm not sure what will happen...I know I almost never have access to preview attempts, even though moodle gives a grade and feedback. I'm not sure previews are included in the numbers...