FIX: Addons not loading with content packs installed and in addon folder

If you (like me) have found that you have all of the content packs installed and the addon in the addon folder but it does not work, then this is a very short and quick answer.
1. check that you are subscribed to the correct content
2. check that you downloaded the addon correctly and that it is not a .zip
3. check the name, on most linux based systems (servers mostly run linux) capital letters will not work, so replace all capitals with lower case characters
4. restart your server
and if all else fails, contact the developer or whoever you got the addon from.