I've sometimes found that some server graph RAM / CPU / Traffic is missing on the dashboard. So, I can't view the graph at that time.

But when I check on the next time, that graph shows again.

What caused it and how to solve it ?

My detail specs:

Device Monitor: 400++ VM (various OS Linux & Windows)

CPU: 8 Core

RAM: 12 GB ( Use Ramdisk )

Storage: 500 GB on SSD

Observium 20.1.10233 (20th January 2020)
OS Linux 3.10.0-514.el7.x86_64 [amd64] (CentOS 7)
Apache 2.4.6 (CentOS) OpenSSL/1.0.1e-fips PHP/7.0.19
PHP 7.0.19 (OPcache: ENABLED)
Python 2.7.5
MySQL 5.5.52-MariaDB (extension: mysqli 5.5.52-MariaDB)