Stick up Camera motion detection temperamental

I have just bought a stick up camera and placed it above my front door. It picks up motion as people are walking past my drive but not when anyone is entering or leaving my drive. Tried adjusting the motion settings, but it makes no difference. Is my camera too high ? I would say it is about 7 or 8 foot above ground level. It is aimed as low as it can go. Any help would be great.