This is similar to the way that real-life radar works, so I'm going to assume the issues here are the same.
The problems with scanning for objects at very low altitude is that most such scanning devices work be sending out a signal, which bounces off distant objects and returns to an antenna/receiver. At low altitudes, there are a lot of things that get in the way of this signal working properly:
- Uneven terrain often means the signal bounces off mountains, etc. and cannot "see" past them
- Lots of natural objects (note that the Falcon is flying through trees at one point) confuse the signal and "hide" objects inside them
- The geometry and configuration of the broadcast and receiving antenna are likely optimized to look for objects coming in from far away -- in space, for example -- which may leave "blind spots" near the ground.
(Modern radar is getting much better at eliminating these blind spots, but the other principles of interference and terrain features still apply.)