It’s common, that several flights are not monitored on Flightradar (where your link goes to). Flightradar filters several aircraft for different reasons (e.g. Military flights).
I see pretty often flights/aircraft on my local map, but they are not shown on Flightradar.
Other interesting aircraft are shown, e.g. these ones:
They belong to “Gesellschaft für Flugziel-Darstellung”.
These are often used to display training targets for other military jets or ground-to-air defense systems.
Not sure if i would feel well in such an aircraft knowing that i am the target of an air-defense system