Failing to install certain addons to my srcds

Before asking let me get this out of the way:
Srcds is properly installed and updated.
It is working, no errors what so ever.
Port is open, people join, you get the point.

I’ve attempted to install the stacker tool, but it does not appear in-game.
I installed it in the addons folder "C:\srcds\orangebox\garrysmod\addons\stacker.

What did I do wrong?
The same happens to advanced duplicator and yet other tools work just fine.

Anyone know how I can fix this?