Was NW Trails the last map you sent to the GPSr? Did you send it individually, or with other maps?
If it wasn't the last, you may have overwritten it with the last gmapsupp.img. BaseCamp writes a file called gmapsupp.img everytime you send a map. Since the installer for NW Trails is only 32 MB it's not likely that NW Trails is taking up more than 40 MB on your GPSr. So, if you sent it separately, rename the gmapsupp.img on your GPSr to Somethingthatmakesmoresensetoyou.IMG and send NW Trails again. If you combined it with a few other maps in the 544.1MB gmapsupp.img. Try re-sending each mapset separately, re-naming gmapsupp.img after you've sent it.