Hi Daniel, there are several reasons why the VPN might not work.

I suggest you take a look at the logs. You could try running Forticlient from the terminal with 'forticlient gui' and wait for the problem to occur. You should see an error with additional info.

You could also go to Settings > Export Logs and try to have a look there.

I have had similar problems in the past because I was using a version not supported by the server.

Hope this helps.