It might be a dirty sensor. The parking assist uses sensors in the bumpers. Give them a good wipe down they could be covered in road grime, salt, or ice blocking the signal. If they're clean and it still doesn't work, it's likely a sensor has failed and needs to be diagnosed by a shop.

 

Parking assist relies on detecting other vehicles to park in between; it does not work with lines on the parking lot or grass sides of a driveway. There are videos on YouTube of several people testing/demoing how it works.

For your driveway, you might try the memory parking feature. This relies on GPS to duplicate a parking sequence that you teach it. It uses the LIDAR to detect obstructions but not to find its programmed space.