Loose windshield rear view mirror on '02

I had the windshield replaced and the rear view mirror is loose. Service writer at the dealership demonstrated his technique to tighten by pushing the mirror down onto the window mount. In a few days is loose again and I can do no better by myself.
Any ideas appreciated.

Is there a snap cover on the mirror base. Most rear view mirrors have a set screw to tighten them to the windshield mount.

Nothing under the cover but the P.C. board for the compass and some cheap plastic. It likely snaps or presses into place but I don't want to damage it.