Plex only loads the 3rd party Agents when you restart the server, Plex also needs to have permissions/ownership over all related files in the Agent.
That were are least the most common support questions on why my own Agent (MyAnimeList) wasn't added to Plex.
I would also look into the log folder, there should be a log being created com.plexapp.agents.amsa.log
or something similar. This would tell you that the agent is being loaded.
Another thing to look for is the general Plex Media Server.log
which would throw errors when the Agent is not being loaded because of an error.
Those Agents also don't work with certain Plex features, like the Watchlist and, when you have a library created with a Plex official Agent, you cannot use the Agent in tandem so matching one Show with a 3rd Party agent doesn't work. You need to have a dedicated Library for that particular Agent.
Lastly, keep in mind that Plugins (which Metadata Agents are falling under) were deprecated in 2018 so weird behaviour is to be expected.
I would first check the Log files that they are created and that may be the Main log file mentioning that the agent is being loaded. If that is not happening, check the Permission and ownership on the system and set it correctly. And if that is all correct, maybe a few reboots of Plex will help.
I had a few VERY strange behaviours happening with Plex reported in which my Agent was loaded for one Library and not another even though they are in the same file...