you try everything and nothing works.
This is why being able to have an onsite nanoVNA measure at the antenna may be the only definitive answer.
The nanoVNA does not lie BUT you must ensure ALL the Calibrations are spot on.
I had a case recently where the trace was jumping all around and no matter what I did I could not get a proper reading.
Until I remembered, the THRU path was not calibrated. there are cases when the other port is seeing something, and ensuring the thru (port to port) is calibrated the device will only tell you what it is seeing and if you have NOT calibrated out ALL the extraneous anomalies your measurement will be compromised.
VE6LX