that the range you are trying to receive is not locked out.
when you are in close call only mode it will say close call on top and have 7 bars and numbers
they represent the bands that can be locked out or unlocked
1 is 25 to 54
2 is 54 to 108
3 is 108 to 137
4 is 137 to 225
5 is 225 to 320
6 is 320 to 512
7 is 758 to 1300
if there is a number then it is unlocked if there is an X it is locked out and you shoud not get anything in that band.
also there is a difference in reception range as the frequency changes
of course the other question is how do you know that you can't get something if you can't hear them?
To expand a little on what mule1075 has already posted....
Make sure you don't have CTCSS/DCS set up as the default on Close Call. That causes the scanner to look for PL tone or DCS code, and only look at analog signals. Digitals are ignored. If you want to search for both analog, as well as digital hits with Close Call, under Search/Close Call Options (from the main Menu listing), be sure that Tone Search is set to Off. If you only want Digital, select P25 NAC Search.