What we've found to work for most is the following:
Clearing the Epic Games launcher cache
Press Windows Key + R and type in "%localappdata%" to open a File Explorer Window
Look for the Epic Games Launcher folder and delete it
Restart your computer, start the Epic Games launcher and try the game again
Reinstalling the launcher
1. Uninstall the paradox launcher via Add/Remove programs (not EPIC)
2. Restarting your computer
3. Reopen the game so that is forces the latest version of the launcher to install
4. Scroll down the dlc list to the left you should see your installed dlc at the bottom with checkmarks next to them
If those don't help you should probably contact Epic support - it's their job to get items you bought from their store into your Epic account, and then into the Epic client.