My gut is saying it seems like you are only getting the 1090 and not the 978 UAT. I'm not sure on the GDL-39 data path as it's a bit non-standard for us but it should be a dual band receiver. You might go check what "ADS-B In Capability" you have selected on the TailBeacon. Changing between 978 and 1090 may get the data coming in more reliably.
I'm guessing this will be found via SET->CAL. Can you please give me the rest of the path to this setting?