First thing I learned (the hard way) is that for whatever reason, these things don't like manually entered system settings. When I entered local Ohio MARCS-IP sites manually and put in the talkgroups, I heard very little. However, when I used the Library Import feature ("Preferred" rather than zip code or location based), selected the specific sites/talkgroups and downloaded it, it worked like a champ. Never have figured out why this is the case; control channels, talkgroups and settings were all identical each time, but it just did not like manually entered systems.
The biggest problem with the newer GRE/Whistler scanners when all the sites are entered as a single multi-site system, the radio finds a control channel it likes at the moment and stays with it, even though the scanner can hear multiple sites and the one it's on may not normally be the best (or doesn't usually hear the talkgroups you want). This is essentially the same behavior you get with the PSR500 set to "roam" in the system settings.
I've developed a process that works pretty well based around scanning by county and entering each county (or simulcast site) as a separate "system."
The first thing I do is start with a blank Virtual Scanner, define the coverage area I want for it, and name that Virtual Scanner as such. Then, download all the sites and talkgroups for the system in that coverage area, get all the scan lists and miscellaneous settings (backlight, LED, decode levels, etc) set up the way I want, and name it for the overall system (for example, Ohio MARCS). Do a "save to archive" so you have a clean backup.
Then, copy that system and name the new one Cuyahoga County (using the Cleveland area as an example), and lock out all except the Cuyahoga County simulcast. You can leave the scan lists alone or delete all but the ones for desired agencies in Cuyahoga County. I usually leave scan lists alone so I have a better chance of catching talkgroups which may have roamed away from their usual site.
Next, copy the "base" Ohio MARCS system again, name the new one Lorain County, and lock out all except the Lorain County sites. You can leave the scan lists alone or delete all but the ones for Lorain County agencies.
Repeat this process for each site (or group of sites) you want to treat as separate systems, locking out all but the desired sites for that "system."
Finally, lock out the base Ohio MARCS system, unless you want to use it as a catch-all. In Ohio, I generally leave the "Ohio MARCS-IP" entry unlocked, but lock out all the scan lists in it except one which has the talkgroup Wild Card, so I can search separately from the designated scan lists.
This way you've minimized the amount of stuff you have to type in and the radio will scan each site separately, giving you a much better chance of hearing what you want to hear; when traveling, you can enable and disable scan lists as needed to scan what's around you.
You'll probably also need to fiddle around with the decode settings on the Advanced tab; after I tweaked the DSP Level Adapt setting to 100 (after much experimentation), P25 simulcast decode is almost flawless.
I think you'll find yourself hearing more stuff, more reliably. Works great on a TRX-1.
Edit: Forgot to mention that you can copy/paste talkgroups and it will work fine. It seems to be just the system-level stuff like sites it's sensitive to where doing Library Import works and manual entry does not. You can just import sites and a talkgroup wild card and get by just fine.