DVB-T ADS-B receiver and feeder problems; advice and assistance please

I have an existing antenna setup for scanning. It is a 2m discone approximately 15m above the ground. It is rated for frequencies between 25MHz and 1300MHz.

This is connected via low loss 50 ohm cable to a wideband signal pre-amplifier at the point it enters my flat. This is a 25-2000MHz 6-20db (non-adjustable) wideband pre-amplifier.

In turn, this is connected to a screened socket, to which I connect a wideband scanner. I use this system for listening to various broadcasts, including aviation radio etc. and ham radio, and it functions well. Signals come in strongly over a variety of different frequencies and modulations.

I thought I would give it a try with the DVB-T Stick ADS-B receiver, using zadig, rtl1090 and the instructions provided. I know that a discone is not the best antenna compared to a 1090MHz specific one, but thought it would work OK. I bought the stick and made a BNC to Belling-Lee converter, and I have tested it - the conductor and the screen have good integrity throughout:

All looked fine.
Here's the startup log from RTL1090:


Here's the output of the "max/min" button:

Here's a minute or two of log data:


I am however facing issues. Please see the screenshot below:


I've tried multiple times; re-booting, selecting correct ports, using tuner auto gain control or setting the gain manually, and so on, and have run the system for days. But I experience the same issues.

1) RTL1090 only ever "sees" a very limited number of aircraft.
2) the FlightRadar24 feeder software doesn't ever see more than one airplane.

I am at somewhat of a loss as to what is wrong or what to do about it. Any help or advice would be very gratefully received.