Why does my OP25 setup skip half the talkgroups on my county's P25 system?

The issue here is almost certainly tied to the operational mode of the OP25 software, not the signal strength, given that you are successfully receiving and tracking the control channel and a few talkgroups. Most trunking software defaults to a limited set of IDs unless explicitly told to monitor the entire system. You likely need to configure the software to run in a comprehensive 'System Scan' or 'Full Talkgroup Monitoring' mode rather than a simple 'Target Talkgroup' mode. When the software locks onto a talkgroup like fire dispatch, it registers that specific talkgroup ID and begins following it. If that mode is active, it will only jump to other groups if the system controller forces it, or if it manually detects a change in the primary assigned group. Since you are missing public safety groups that are clearly on the same network, the software isn't actively polling the system for the complete list of assigned talkgroups. Beyond the software settings, double-check the system definition you have entered. Some trunking systems require specific parameters like the number of available sites or a unique system identifier that isn't just a frequency. If that definition is slightly off, the software will only successfully track the most prevalent or highest priority talkgroups it recognizes. Regarding the hardware, while a weak USB stick can definatly impact performance, expecially when trying to jump between multiple channels, the fact that you hear clear voice on the working groups suggests the basic RF path is sound. The limiting factor is the software's ability to index and listen for the non-primary talkgroup assignments from the controller.

I agree with the operational mode point, but if the software is still missing groups, check the network configuration itself. Sometimes the issue isn't the scanning, but the system definition not fully matching the controller. When I mapped out a major regional P25 system, I found that setting the maximum number of sites to 12 often forces the software to index more of the database than if I just left it on a default of 5.

Also, verify the trunking algorithm setting. Some older deployments use a proprietary scheme that certain generic scanners don't handle well. If the system uses a specific type of talkgroup structure, make sure that setting is explicitly enabled in the preferences.

I ran into this exact scenario once with a diferent county. The software configuration was set to a comprehensive scan, so I spent days looking at the software settings before I realized the talkgroup list itself was just out of date. Changing my entire frequency database was the only thing that made the missing channels appear. It turned out the problem wasn't the way the software was monitoring, but that my source data was several years old.