Is the Addon Manager reporting the sceneries to not be installed in the Scenery Library ?
Have you tried to add them to the Scenery Library yourself, trying to fix the problem ? If you did that, they won't work, because you created a double entry, and a double entry for the same scenery will make it invisible.
Try to Uninstall everything, remove any scenery areas you might have added manually, then download all the updated installers for all the FSDT sceneries and install them only from the updated installers.