Are you sure you have waited for the radar to warm up?
From page 129 in the manual :
Therefore,you can turn the RP-22 radar main mode (RV62) to stand-by when your aircraft is ready for takeoff. Usually, it will take between 3 and 5 minutes for the radar to warm up. After the warm-up time has passed, you can test the radar using the RP-22 radar self-test button (CM51).