The obvious thing to do is to swap antennas (by rolling their coaxes) between one of the Nanos and the FlightAware stick and see if the problem follows the Nooelec antenna or the FA dongle. If the Nano with the Noolelec antenna has the same issue, it’s the antenna or the coax. The next step would be to roll the coaxes back at the dongles and roll them at the antennas instead to see if it’s the antenna itself or the one coax.
If the problem stays with the FA orange dongle, it’s the dongle, and as it contains an LNA your next step would be to experiment with reducing its gain in steps to find its sweet spot, unless you’re using dump1090-fa and have adaptive dynamic range enabled. If changing the gain doesn’t help, add a filter near the antenna and see if that helps (after setting the gain back to what it was.)
You can do the same thing with your Pis: swap dongles between the one that now has the FA stick and one of the others with a Nano in case it’s a Pi issue (which I think is unlikely, but anything’s possible.)
It’s just basic troubleshooting: sectionalize the problem, sectionalize it further within the bad section, etc. until you isolate the cause. If the problem comes clear during your troubleshooting changes it was probably due to a bad connection, either coax or USB, and you fixed it while changing things around.