Be sure to look here for Trunking Basics...
http://wiki.radioreference.com/index.php/Trunking_Basics
My crude definition... instead of each police division/fire department etc getting thier OWN frequency... they share a few common frequencies (say 20)...
Then when the police division becomes active the system gives them a "free" channel (of the shared ones) to communicate on.
So you cant monitor them on just one frequency. You need them all programmed in, then the TalkGroup that the divisions you want, in order to hear them when they swtch frequencies.
Which scanner are you using? Sometimes software can be 10 times easier to program with.