Let's first go over what may be coming across as updates. A full map update for your unit will be BIG. Something more than a gigabyte. The small regular updates you receive are updates to whatever map is already installed on your unit. If you purchase a subscription, you get the full map replacement update quarterly. Even with no subscription, we all receive Mapshare updates give or take weekly, perhaps more often. Those are incremental improvements to older maps that eventually find their way into newer full maps as well.
If you purchased a subscription, when did you purchase it, and have you ever successfully downloaded a new map for your unit? Again, we're talking something BIG that would take you more than an hour to download and install.
To see if you have a newer map (newer than 875, anyway) on your PC, check in the following location for a folder called "North_America..." or "USA___Canada..." or similar.
My Documents / TomTom / HOME / Download / complete / map
Let us know what you find there.