1. Are the aircraft from the same reception category, that is, are they all ADS-B? Or is there a mixture of PAW(P3i), FLARM and ADS-B going on here?
2. When you changed SDRs did you run the set up again to calibrate the antennas?
3. Without knowing what your ATOM web page is displayed on, can you check that you haven't inadvertently set any filters on the display? What is the VRS running on? Phone?
4. If you are using a PAW with just the small stick that came with an SDR and no other antennas, you are only receiving one group of signals, either ADS-B/mode-S or PAW.
5. If you suspect the ground station is not working, you can log into it and select one of the menu options to see if you are receiving FLARM or ADS-B traffic.