One of the most common causes of addons not working is the folder name syntax.
Garry's Mod only accepts addon folder names with lower case, alphabetical names. That is, folders with capital letters, spaces, and special characters will be ignored.
If you have addons that are not working, check the folder names in the File Manager.
Some addons are OS specific. Most Garry's Mod servers with Wombat Servers, unless you have arranged otherwise, are hosted in a Linux environment. You should always install the linux distribution of addons as Windows specific addons will not boot.
To further troubleshoot addons, check the web console for errors. It is best to restart the server with the web console open in order to see if the addon is being mounted to the file system.
Ultimately, if an addon still does not work, you are best to contact the developer. We cannot support software published by third parties; the developer of the addon will be able to offer you the best and fastest assistance.