app.4dSky.com does not show any planes at all neither from my sensor nor anyone else

The main reason this can happen is when you have the website open on two tabs in your browser. Then only one will have planes.

Other reasons are the website reboots and and the consumer needs to re connect to all sensors and this takes some time. Other reasons that are very common is that the consumer runs out of test hbars and needs a quick refill.