The Mexican maps I got in the download are detailed to the same extent as they are in the USA and Canada. You have to be careful when you choose what you ask for in the download as there are three or four choices as I recall. Maybe you should call customer support. They will send you to the correct site.
The non-detailed map you describe was what came on my Garmin even though it said "North America" I corrected this and got the full map at the time of my first update. I did however, have to add an SD memory card in order to create enough memory for the new map file.