You mean some radars inability to detect stationary obstacles. Clutter rejection has a lot of more sophisticated algorithms to apply with greater compute power to throw at the problem.
These were flat truck/ambulance surfaces encountered at an angle - exactly the conditions that the first stealth fighter, with its angular surfaces used to evade some of the best radar in the world, because from most angles, no radio waves would be reflected back to the radar sensing device. It's hard to get the job done with nothing.