I’m trying to use a custom component which is reported to work by others. I copied it to the directory .homeassistant/custom_components but seems to be ignored, the integration cannot be selected in the settings menu.
I have a core installation (cannot use HACS or Addons)
What could be the reason the custom component is not registered?
Restart and check the logs and test if it then works.
Custom Components always require a restart.
I followed the instructions, which included a restart, even did a reboot of the server, but no effect, unfortunately
You need to clear browser cache as well.
However note that not all (especially custom) integrations does not support UI configuration.
Cleared cookies, restarted browser, and logs do not show any reference to the integration. The maintainer confirms no manual configuration is required.
What integration?
And have you cleared you browser cache for the HA integration store?
Sometimes the browser cache for the HA integration store is many days or months old and some browsers are configured to only clear the cache from the last 1 or 2 days.
And cookies are not cache. 
And share exact folder structure
If the logs have no refrence of them, the files are wrongly located or have wrong permissions.
It is the coway iocare integration for controlling coway airpurifiers.
I cleared all cookies and cache from the server on which HA is run, also tried to find the integration in a private window (Firefox) and from my mobile phone.
The integration folder is (from root)
/home/ha/.homeassistant/config/custom_components/coway
The file permissions of the files in the integration directory are u+rw, go+r (owned by ha:ha, which is the proces owner)
Do you have any other custom components installed and do they work?
If this is your first custom component, then it could be a missing entry in the configuration.yaml, like default_config:
I have no other custom_components installed.
The default_config: enrty in my configuration.yaml,is present but there is actually nothing in it …
Found the solution.
I moved the integration folder from
/home/ha/.homeassistant/config/custom_components/coway
to
/home/ha/.homeassistant/custom_components/coway
After restart homeassistant the integration was found.
Thanks for all help!