NOTE: This is NOT a problem caused by the mod, it is a known issue with Paradox's launcher and has existed ever since the new launcher was released. Paradox, in their infinite wisdom, have never fixed the bugs with the launcher.

If the mod doesn't show up in your mod list, or if you try to play the mod and nothing from the mod shows up in-game, then try the following steps. You don't necessarily need to do every single one of these steps. After each step, try loading up the game and see if it is now working correctly.

1) Unsubscribe from the mod and resubscribe to it. You may have to attempt this several times. This will fix the issue for the vast majority of people.

2) Unsubscribe from the mod and uninstall HoI4. Then reinstall HoI4 and resubscribe to the mod. It is very rare for this problem to persist after this point.

3) The nuclear option. Complete step 2 again, but before reinstalling HoI4, go to Documents\Paradox Interactive\Hearts of Iron IV. Delete the mod folder. Reinstall HoI4 and start a game with no mods turned on, this should recreate the mod folder that you previously deleted. Subscribe to the mod.

4) The ultra nuclear option (NOTE: THIS WILL UNINSTALL ALL OF YOUR HOI4 MODS AND YOU WILL HAVE TO RESUBSCRIBE TO THEM AGAIN). In your steam app, click on Steam at the top left of the window and select "Backup and Restore Games". Create a backup for HoI4, then uninstall HoI4. Find the game folder in C:\Program Files (x86)\Steam\SteamApps\common\ and delete the Hearts of Iron IV folder if it's there. Then go to Documents\Paradox Interactive and delete the Hearts of Iron IV folder if it's there. Reinstall the game by again going to "Backup and Restore Games", but select Restore Backup this time. This should ensure that when you reinstall the game, it will be as a blank slate.