IVS, and intrusion boxes, can be problematic at times. Usually it's a problem caused by contrast or not enough time (distance travel) of the object for the camera to see and identify it well enough to understand it should detect it. In your case, try an intrusion box, but again remember the camera needs some time to "see" the object to detect it before it enters the intrusion box. That requirement can be a real headache in tight views.