If you haven't already its worth dropping us a ticket:

https://support.justflight.com/support/tickets/new

If FSX is showing in the list but won't change when enabling it:

It sounds like you may have your scenery.cfg file set to "read only" and thus cannot be changed by the program.

You can try right-clicking on the cfg file and select “Properties” and see if the “Read only” option is ticked. If so un-tick it. The manager should then be able to activate it normally

Thanks,

Simon