Hi, a few possibilities.
The sd card should be formatted to fat32, the file should be unzipped and gmapsupp.img (or you can rename to something unique) and in the garmin folder.
Have you gone to the setup page > map and ‘enabled’ only the new map on the ‘Map information Select Map’ tab.
Also, be careful that the sd card is locked in by the latch correctly as a loose latch can shut it down while in use.