ADS-B, receiving incomplete TIS-B traffic

When I first got my Navworks ADS-600B it seemed to be showing most of the traffic most of the time. Lately, that's dropped way down. Now I'll be flying in my "normal" area where there's decent radar coverage. For example, screen shows 4 targets, and the system display says it sees two ground stations, two ADS-B (direct) and two are TIS-B (rebroadcast) targets. Meanwhile ATC is calling out multiple targets to me that aren't showing up for me.

Either TIS-B isn't rebroadcasting all radar returns, or TIS-B isn't rebroadcasting to me based on my position. How can I tell if the system is even functioning, or who would I ask?

I'd think they were refusing to rebroadcast to me because I still have SIL=0, except that's not supposed to happen until 2016 -- right?

This sure isn't giving me a lot of confidence in the system.

The ground stations send out a packet that tell you if you are in TIS-B coverage. I don't know what app you are using to display traffic, but look in that app for a status that shows that. I think it's officially called "client status" in the ADS-B Protocol.

On SkyView, this is what drives the ADS-B FULL / ADS-B partial in the corner.

Question: will the system send out info on non-ADSB-out, non-mode C (e.g., mode A only) aircraft?

Yes.

From the AIM, section 4-5-8:

b. TIS-B Capabilities.

2. Only transponder-equipped targets (i.e., Mode A/C or Mode S transponders) are transmitted through the ATC ground system architecture.

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