at a ZCM 11.4.3 I got the last PRU in August 2017. Since then I get no
new PRU. If I click 'Check for Updates' in ZCC/Configuration/System
Updates I get "There are no new system updates available."

I am able to download the updates by hand and copied them to the server
as described in the documentation. But at least I know this should
happens by itself?!

What's should I check?