If you reinstalled P3D with a version less than 3.4, the installer will still use the old scenery.cfg method, rather than the new add-on.xml (which will result in areas greyed out).
But if your previously installed using P3D 3.4 and now are reinstalling P3D 3.3 or less, the sceneries will both not work and won't auto-discover, because previous versions had some bugs in the handling of the add-on.xml system. Which is why, we don't use it in versions before 3.4.
Basically, you should either be sure you install the latest version of P3D 3.4 or, even better, V4.0.