I have a rugged mount with the same problem. The security screw will not engage, no matter how much I work it, make sure the mount is closed, etc. It just won't engage.
I bought mine from GPSCity, and emailed them to let them know of the issue. They got Garmin to send me a new rugged mount free of charge.
+1 for GPSCity!
Successfully surviving motorcycling since 1976.