Missing flights

Hi
a friend has recently installed a new opensky sensor in Pantelleria. While looking at the opensky explorer map in this area, we noticed that some flights are missing.
To give an example, the TUX301 flight seen in the attached screenshot from FlightRadar24 was never reported by the OpenSky Explorer:





I have been checking for several minutes the OpenSky Explorer maps, and this specific flight NEVER appeared there, while instead it was regularly tracked by FlightRadar24 along his trajectory. As the flight was not on the ground, nor there is any obstacle between the (newly deployed) sensor and the aircraft position (as reported by FlightRadar24), I would have expected that at least some message from this flight would be seen by OpenSky.

This is not the only case: we have seen a few other similar cases in the same area (I haven't checked for other areas), e.g. flights from Mistral Air between Pantelleria and Sicily seem to be also missing.

Could anybody propose a possible explanation?
Perhaps it has something to do with the message filters (?) and perhaps it points to a problem that can be fixed ?

The flight is missing in our explorer since it does not report its location via ADS-B. Flightradar uses multilateration (MLAT) to locate these aircraft (indicated by the radar name "T-MLAT..." in the bar on the left).

Currently about 70% of the commercial air traffic in the US and Europe is reporting locations via ADS-B. The other 30% are not shown in our explorer yet, but we collect all data needed to do multilateration and will include it as soon as possible.

More information about the current ADS-B equipage and usage is available here:
opensky-network.org/files/publications/dasc2016.pdf

Hello,
I'm comparing all the flights arrived per airports since december 2017 between opensky and flightradar, sometimes differences are huge like 100 K flights are missing sometimes from opensky, sometimes from flightradar, is there any reason ?
Thanks for your help