Maps confusion-not storingdownloading caching data iPhone

I have been travelling abroad for the last 2 months with cellular data on my iPhone switched off. Before I leave each location I have been connecting via wifi and searching for and viewing a map of my next destination on the iPhone Maps app. I discovered that each time I did this, Maps retained most of the map data that I had viewed (in the cache I presume?) which meant that once I arrived at my new destination I could view the map offline (no longer connected to any wifi and still with cellular data off) and the GPS would pinpoint my location - very useful. Of course, I cant search for anything or change the level of zoom much but it has been extremely helpful.
As of 3 days ago this has stopped working. Can anyone think of a reason why this might no longer be working? All Ive been using it for is the iPod and connecting to wifi hotspots for email, web browsing and, of course, Maps. Since 3 days ago, when I am connected to wifi it allows me to search and browse maps but as soon as I close the app and re-open it the I get a grey screen while Maps downloads the data again - if I am offline then it obviously cant download it so it doesnt work.
I would appreciate any help as the ability to use Maps like this has been very useful for us in a foreign country.
Many thanks,
Alasdair
iPhone 4 iOS 4 iOS 4.2.1