Pulling my hair out here.

I have a Nuvi 755T
I can't get it to read the 2015 map IMG file on the SD card.
I've tried two different unlocked IMG files (CN North America 2015), one just USA & Canada (1.87 GB)and one the complete IMG (2.16 GB)
I've checked that the file name is gmapsupp.img
I've checked that it's in a folder on the SD card named Garmin (I also tried all lowercase garmin)
The SD card is a brand new 8GB micro SD.
The card works fine in the PC, and when I connect the Garmin to the PC I can copy files back and forth between the device and the SD card.
The device will play MP3 files stored on the SD card.

But whenever I power up the device and go to Tools>Settings>Map>Map Info, it only shows my old map file on the device (2012)

I have updated the device software with Garmin Express
I have reformatted the SD card (it was already FAT32 but I did it again anyway)

Any ideas? Thanks