So what can be wrong?
Reinstall GSX, better if you use the current installer, then enable the Simconnect diagnostic:
Open Notepad, and copy the following text:
[SimConnect]
level=normal
console=1
RedirectStdOutToConsole=1
OutputDebugString=1
; file=c:\simconnect%03u.log
; file_next_index=0
; file_max_index=9
Save the file as SIMCONNECT.INI in this folder:
Documents And Settings\YOUR LOGIN NAME\Documents\Microsoft Flight Simulator X Files
At the next FSX launch, you should see a text window with diagnostic messages: the FIRST 10 lines or so are important: you should try to note what they are saying. If the text scrolls too fast, use the Pause key on the keyboard.