Remember, in a wide area trunking system, a unit has to be affiliated with the tower you are listening to with their radio set to a specific talk group in order for it to transmit on that tower. If a unit is somewhere the radio decides another tower site has a stronger channel it will jump and you won't hear them.
This has been a common complaint on the Kansas system. Not just for scanner listeners, but for end users. Here is an example, two troopers are working a neighboring counties and both are operating on the same talk group- let's just call it Dispatch. Their car radios are affiliated with different towers, but when Dispatch keys up the controller knows that there are units affiliated with each tower and broadcasts over both of them. They also get to hear each other talking on that talk group to dispatch.
Now, one of the troopers decides to call the other trooper to schedule a break. To keep the Dispatch talk group clear, he switches to the car to car talk group and calls his partner. When he transmits, the controller sees what towers someone has a radio set on car to car and transmits over those towers... But his partner does not hear it because no one on the tower he is affiliated with has a radio set to car to car. As a result, you will usually hear units calling others on the dispatch channel to switch to another specific talk group so that they can talk to one another.
If you have a multicasting subsystem throughout a county, you can usually avoid this. But with a single site it is easy for units to jump to other towers as they move around. I have had a unit stand next to me and affiliate with a different tower.
