Mercedes e class 2004 sat nav problem?

I have the above car which has a sat nav problem some days it will not read the sat nav disc. Some days its fine but on occasions the screen says no disc inserted. I have tried a couple of discs to make sure its not the disc but it does the same. I purchased a disc cleaner with the two little brushes which seemed to do the trick but after a couple of weeks it is playing up again. I didnt know how long to leave the cleaning disc in so I left it until it said no disc inserted. Have you any ideas what it wrong.

A Walker
July 2011