Flight Simulator X Add-on planes not working (black screen o

To make gliding a little more realistic in FSX I installed WinchX and CumulusX, and added a few gliders to fly with. Now only the DG-808S is working, while any other add-on aircraft will just result in a black screen that loads forever. They are installed correctly, and CumulusX and WinchX both work perfectly with the DG-808S. I've already reïnstalled the game and completely removed and reïnstalled the add-on planes, with no success. Alt-tabbing out of this black screen after a minute or even an hour always results in the game that stops working.