John,
You are correct. An ADS-B receiver will only see aircraft either being reported from the TIS-B Ground tower or aircraft with ADS-B out on either 978(GA) or 1090(Commercial). The TIS-B Ground Broadcast is only active when there is an ADS-B out plane that has trigger that Service Volume area to "wake up".
I've run numerous test now with Clarity, iLevil, SkyRadar receivers and the SkyGuardTWX transceiver. Example, I can have the iFly and a receiver connected, seeing only a few traffic targets. Once I turn on the ADS-B out (978), within about 30 seconds those other ADS-B receivers light up like a christmas tree. That is here in the DFW area.
May sure you have 7.6.7 as the software version as some of the earlier versions only showed traffic that was in alert status. That has since been disabled. We are speaking with SkyRadar to get a D2 model to test and see if anything different is going on. I believe they do use a slightly modified code from the GDL90 standard.
We highly stress that ADS-B receivers are really only recommended for Weather. If you are wanting Traffic, you really need an ADS-B out Transceiver to wake up the towers. It's my understanding that an airliner with 1090/1090ES out, won't wake up the 978 TIS-B ground tower, but I've not been able to confirm this. The towers are woke up by the 978 out.
Norman, we probably recommended the SkyRadar L-model at the time based on connectivity and ADS-B Weather. I would be supprised if anyone here would recommend a single band receiver for traffic applications.
ADS-B traffic is still very limited and not a replacement for TCAS or something like a ZAON. It is getting better, but I fly very cautiously with a ADS-B Transceiver. There is alot of confusing information out there on ADS-B traffic, I suggest looking at sites like the FAA and non receiver only companies as they are not biased.