Last year I started having the same problem with my Garmin 2557LMT. Emailed Garmin and got the usual boilerplate things to try. None worked.
Found 2 ways to get it to work:
a) Do a reset on the unit, which erases all waypoints but not the map. Problem eventually crops up again, though.
b) Connect it just after rebooting my laptop, but before the boot is finished.
I'm convinced something is wrong with the unit. I've almost filled the 1000 waypoints storage. Maybe this has something to do with?
Probably will buy a new unit before I hit the road this summer. Won't buy a new Mac (or PC), though. :)