Google maps doesn't know where I live?

When I click on the gps button in google maps it always points to a different location to where I am. For example I try it when in my back garden and it says I'm a couple of miles away from where I actually am. But when I try this in the google earth app it finds me straight away in the right location. I then go back to google maps and it now pointing to the right location. Very strange! An ideas?

Cheers