Trying to update from 15.00 to 15.20.

Good luck. Macs are definitely second-class citizens in Garmin-land.

There are at least 2 possibilities. Garmin sometime releases new f/w in waves. Might be that your unit is not yet eligible. Second possibility is that Express (do Macs use Express?) has dropped the file on the unit. As far as Express is concerned, you are now up to date. Even tho not yet actually installed. You should be prompted to install new f/w on the next boot.

if neither of those seems to fit, open a support ticket.

Edited to add: Given the amount of time since release, phased release seems unlikely to be problem now.