My guess would be either a faulty sensor, or the sensors are covered by muck, ice or snow.Your handbook should have a section on the reversing sensor operation. It is section 4 in my Sportage handbook and it says:-
Non-operational conditions of parking distance warning reverse
The parking distance warning reverse may not operate properly when:
1. Moisture is frozen to the sensor. (It will operate normally when the moisture has been cleared.)
2. The sensor is covered with foreign matter, such as snow or water, or the sensor cover is blocked. (It will
operate normally when the material is removed or the sensor is no longer blocked.)
3. Driving on uneven road surfaces (unpaved roads, gravel, bumps, gradient).
4. Objects generating excessive noise (vehicle horns, loud motorcycle engines, or truck air brakes) are within range of the sensor.
5. Heavy rain or water spray exists.
6. Wireless transmitters or mobile phones are within range of the sensor.
7. The sensor is covered with snow.
8. Trailer towing