Not generating ping-perf.rrd on some devices

hope someone has a idea.

For all devices ping graphs (ping-perf.rrd) are generated, except 2 systems.
Both are physical Linux Server which acts as Gateway and have multiple NICs.
My first thought was a firewall issue, but even without blocking anything no graph was generated.

For LibreNMS both Systems are up.
As i see in Sourcecode availability and data for ping Graph will be done via fping.
Fping can ping device. Also seen in debug output on issued devices.
Nevertheless no ping-perf.rrd will be generated.
All other graphs of this devices are generated, Only ping Graph aren’t generated.

Hope someone has an idea.