There are a few things you can do to maximize your detection accuracy.
First, go to General Settings and scroll min face size to the left to capture small faces and max face size to the right to capture bigger faces. If you are running on a powerful hardware, you can select the highest accuracy option (slide bar to right) in the Performance Settings.
In the Input tab you can also increase the camera resolution for instance from 640×480 (default) to 1280×720. If your camera doesn’t support higher resolutions, you can scale the input in Input Settings by moving the Scale slider to the very right.
An alternative to these solutions is to change your camera positioning (e.g. move the camera to have more frontal detections, increase/decrease the vertical camera angle such that it captures more faces).