Is it different in the day vs. at night? Is there any obstruction, like a column or something else? In order for the camera to process an event (Vehicle) on it's local CPU, it has to clearly capture the margins of the object. In daylight, it tends to be able to process the vehicle image better than at night, if it has a more difficult field of view. If the vehicle is distant, daylight also helps it define the object. If there is an obstruction (gutter, downspout, bushes, etc.) it may have trouble as well. Many factors play a part in the image processing, and it may simply be a bad location. Is the vehicle passing by (parallel) or heading straight into the camera (perpendicular).
Maybe try a different place if you can? I have 7 Nest cams, and one of these has the same problem, but I finally determined that there was an obstruction preventing enough problems, and only at night, that it could not identify the vehicle object. People and animals were fine (smaller and it could the whole margin of the object.