You downloaded an addon, followed the steps, and Minecraft either crashed, showed nothing new, or looked broken. This happens to almost everyone at some point, and it is usually caused by one of a small handful of common issues. Work through this checklist in order and you will most likely find the problem.
Bedrock Edition changes its internal format regularly, and addons are built against a specific version range. An addon built for a newer version of Minecraft may not load correctly on an older game version, and the other way around. Open the addon's page and check what version it was tested on, then compare it to your installed Minecraft version under Settings in-game.
Downloading and importing a pack does not turn it on by itself. Every world has its own list of active behavior packs and resource packs. Open your world settings (or create a new world), scroll to Behavior Packs and Resource Packs, and confirm the pack you imported appears in the Active list, not just the available list on the side.
Many script-based addons, especially ones that add custom mechanics, require an Experimental toggle to be switched on before creating the world. If an addon's description mentions scripts, custom UI, or "experimental features", enable the matching toggle in your world's Experiments tab. This has to be done when the world is created; turning it on for an existing world may not always apply correctly.
When you have more than one behavior pack active, Minecraft applies them in the order they are listed. If two addons try to change the same thing, like the same item or the same mob, the one listed lower in the order usually wins and can silently override the other. If an addon seems to have no effect, try moving it to the bottom of the active list.
Some addons are simply not compatible with each other, especially two large gameplay overhauls, or two addons that both modify the same vanilla mob or crafting recipe. Try disabling all other packs and testing the new addon completely on its own. If it works fine alone, the problem is a conflict, not the addon itself.
A partially downloaded or corrupted .mcaddon file can fail to import correctly, sometimes without any clear error message. If your download was interrupted, on a slow connection, or came from a browser other than the one recommended for your device (Safari on iOS, for example), delete the file and download it again.
An .mcworld file is a full world and should be imported as a world, not activated as a pack inside an existing world. Make sure you are not trying to import a world file as a behavior pack, or a behavior pack as if it were a full world.
If you have gone through every step above and the addon still does not behave correctly, it is worth checking whether the issue is on your device or in the addon itself:
Need more installation help?
Read the Install Guide