When doing a zoom or an auto focus in low light scenes the camera may not focus properly due to not having enough contrast to focus with. When zooming a camera the auto focus occurs in order to focus the scene after the zoom has changed.
Pelco Cameras, Pelco Video Management
Sarix Pro series cameras
The focus method uses the contrast between light and dark objects to focus. If the scene does not have enough light to create a good contrast then the auto focus may not work properly
It is best to always do your main focus set up in daylight or brighter scenes