This may be because "Double Pixels" was checked when the maps were downloaded.

An easy way to check this is to:
1. Determine what zoom level you are currently on.
2. Go to "Maps" > "Manage Maps" and select the "Zoom" tab.
3. Whatever Zoom level is checked automatically, should be the level that you manually download for your database.

That is, if you are used to a Double Pixel view and you set up your Zoom Steps to skip every other level you may think you need to download levels 2, 4, 6, 8, 10, and 12 manually.

Because Pixels are doubled, you really need to manually download levels 3, 5, 7, 9, 11, and 13.