Thanks for the quick reply!
The second screenshot was from FlightAware - I was watching the aircraft on their site to know when I should (hopefully) be getting data returned from OpenSky. A few minutes before that screenshot, FlightAware had them at 5,000' at 47.2461, -123.0394 (timestamp 1508472828).
Here's another of our GA aircraft I've been watching tonight -
FlightAware had them at 6,000 ft at 48.6367, -122.5400 for timestamp 1508039895. That's an interesting thought about aircraft being less noticeable at lover altitudes - I didn't know that about ADS-B receivers. You are correct in that a lot of these aircraft are flying at 5,000 - 6,000 feet (4,500 - 5,500 feet above ground level), so maybe that's what's happening. FWIW, here's the query for all our airplanes; I have yet to get data on any of them (but then, maybe there's data when I wasn't looking?).
So, sounds like I may be running into limitations with the network's ADS-B reception, vs. having, say, an incorrectly formatted API call?