What might be happening here are missing ADS-R (R=Rebroadcast) messages from the ground station. ADS-R is a capability of the ADS-B ground station to relay ADS-B messages from one link (e.g 1090ES) to the other link (e.g. UAT) to accommodate aircraft with single-link receivers. The UPS fleet is 100% ADS-B equipped (1090ES). ADS-B ground stations do not include ADS-B equipped aircraft in the TIS-B data stream. TIS-B data is composed of transponder (non-ADS-B) aircraft only. This is to prevent dual targets for the same aircraft. The ground station assumes that you will receive traffic info from ADS-B aircraft "air-to-air", at least if you are equipped with a dual-link ADS-B receiver. If you have a single-link receiver, the only way to receive traffic information from an ADS-B aircraft on the other link is via ADS-R.
IIRC, the NavWorx has a single-link UAT receiver. In order to enable the ADS-R cross-link, or have the 1090ES ADS-B messages relayed to you on UAT, you must have the proper capability bits set in your ADS-B "Out" messages. There are capability bits in your ADS-B "Out" message that tell the ADS-B ground station what type(s) of ADS-B receiver(s) you have. If you have done the FAA ADS-B performance test (
https://adsbperformance.faa.gov/PAPRRequest.aspx), you will find the
"In Capability" in the third section of the
"Operation Summary" of the report. It should state
"UAT". If not, then the ground station is getting an incorrect capability bit (or bits) from your ADS-B "Out" messages. I'm not familiar enough with the NavWorx to know if that can be improperly configured during the installation setup, but something you might check.