Unfortunately weak intensities of precipitation are very hard to visualize. It often occurs that the radar shows area-wide precipitation, but it doesn´t rain.
For example in summertime when the air is dry, the rain drops evaporate and don´t reach the ground. To avoid such situations we filter weak radar echos in order not to confuse the user with a lot of alerts.
Conversly we have the situation, especially in autumn and spring, that the radar often cannot detect the rain when there is a lot of high fog. If such a weather situation is forecasted it can rain the whole day, but on the radar itself there is nothing to see.