Choosing a sensitivity

The slider moves four things at once: how different a pixel must be, how much of a grid cell must change, how many cells must be active, and how many frames it must persist. One control, so you don't have to think about any of them.

The reliable way to set it is with the meter directly below. Have someone move through the scene the way they actually would, and watch where the bar reaches. Set the trigger mark comfortably below that, then check that an empty room leaves the bar flat.

Higher is not better. A setting that catches a moth will also catch every shadow, and a hundred junk clips a day are worse than missing one distant event.

If the top of the slider still feels unresponsive, the problem is more likely your region or camera position than the setting.