I'm going to make a WAG and think you're trying to hear this system...
Linn and Johnson County Public Safety (Project 25) System Trunking System, Various, Iowa - Scanner Frequencies
I would let FreeScan pull the information for setting up the system and site types, since you are already a premium subscriber....
Anyhow, why you might not be hearing much out of this system may well be because of this...
Simulcast digital distortion - The RadioReference Wiki
To put it simply, since both sites are simulcast, this means that talkgroups can be echoed on every tower. If you are receiving the signal from multiple towers, chances are you're getting overloaded - no scanner currently on the market can handle all simulcast systems very well.(The 436 and 536, as I've been reading, are better on some, but not all, simulcast systems).
I would only import the site for the county you live in, and quite possibly, you may actually need to add attenuation on this system (as counter-intuitive as that is). There really isn't any one solution that works for everyone. It's purely trial and error, but once again, the Iowa forum and the Scan Iowa Yahoo mailing lists are better places than this to get information on how folks in your area are receiving that system.
Oh and by the way - anything that has an 'E' in the mode column means 'encrypted' - meaning you'll never be able to hear anything on it (unless someone forgets to turn the switch on, hehe) and the scanner will bypass them anyway
Mike