What happens is that you see a previous zoom level image. The image for the current zomm is corrupted. The file exists but the image is corrupted(might not even be an image). Maemo check for the file name and find the file there so it doesn't download, but cant load image over the previous square, so it doesen't get refreshed. You have to (as you noticed) delete that corrupted file so it will get downloaded. It is a game of hide and seek, but if you move them to windows folder, but if you dump the map cache into lets say picasa, you can see which pictures are not coming up, can't be displayed, and delete those from you cache, after that they will get downloaded again.