There are several common reasons we find that plugins may not be functioning/active on Minecraft servers, so we'll go over those reasons here. If nothing here seems to be relevant to your server, please reach out to our team and we will be happy to help!
You're still running a vanilla server.
Vanilla Minecraft is unable to support plugins, as plugins alter features/items in-game. As such, you'll need to adjust your server type. We typically recommend Spigot as it offers high optimization, however, Paper (a fork of Spigot offering more customization) and CraftBukkit (modified vanilla that ceased operations; overtaken by the Spigot team) will also allow you to run plugins on your server.
You're using outdated plugins.
If you're cherrypicking any random plugins out of the automatic installer and/or pulling random files out of old MinecraftForum threads, you may be installing outdated, abandoned plugins onto your server. Often times, unless the plugin documentation shows that it has been updated to support the version of Minecraft your server is running, you won't be able to run that plugin successfully on your server.
You're not installing required dependencies.
Some plugins require other plugins in order to function. You'll usually see an error in the console telling to you to go install the applicable dependency upon startup, but it's not always that easy. As such, it's typically a better idea to just check your plugin documentation prior to installation to make sure you're not missing anything you may need.
You've installed plugins that are not compatible with each other.
As there are millions of plugins available out there, it can be tempting to download lots - we get it! Before you do, be sure to check plugin documentation and be sure that your plugins will play well with each other. Most of the time, the docs will show if there are known issues with other plugins.