front radars sensor do not detec obstacle infront of vehicle

I was pulling up to my driveway and my front radar sensors were not detecting my fence, to let me know the distance from my car and the fence in front of me. I gots as close 2 inches away from fence and cars radar still did not detect fence. Normally car would detect fence as close as 3-2 feet away. I almost crashed into my fence in my driveway. I took my car to Cadillac and they inspected the sensors and said no issue was found at this time. the only thing that bothers me is would I have to hit my fence before they would rule out some sort off issue is with the car. I was really close to hitting my fence due to the radar not detecting my fences distance. No solution at this time. My car is still under the bumper to bumper Cadillac limited warranty.