Will compatibility be announced soon? It is a relatively new phone so it should be just a matter of a firmware update for the nice not yet old Garmin Forerunner 620?
Best to ask in the Windows Mobile app subforum. As I understand it, the top level forum is for the desktop app.
FYI, while _your_ 620 may be new, the model dates back to September of 2013 and has been retired in favor of the 630. But if an old watch pairs fine with an old version of the app on an old phone, and the same old watch won't pair with a newer version of the app on a newer phone, then the problem isn't likely to be in the watch.