Touratech Garmin III mount - how does it work

I recently bought a second hand Touratech mount for an old Garmin III GPS. I have mounted it on the bike and just tried the GPS in the mount however the bottom spring-loaded clips didn't lock-in and hold.

Am I missing something .... or do I force it in and the pin bits magically hold.

Any helpful adice would be great.

On the bottom of the GPS is the two holes that the pegs fit into and on top of the GPS is the recess that the spring bit clips into.

Fit the gps top part onto the two prongs, then press the gps into the bracket, and make sure the sprung prongs enter the two recesses on the bottom.