Have you tried using the offline installer?
Some people report that the FSDT installer offers a new update, but if the release notes for the update are not visible, the update is not distributed to your node, and that might cause a problem with an update not working.
No other add on for MSFS has this issue of updates being available but not fully distributed, but maybe its because of the huge size of the updates with GSX.
In my case I have not updated because the release notes are still not in synch with the update version more than 24 hours later.