Edit: After further investigation I find that this effect is a result of my use of a previous database , which does not have geocoded places. However the map should appear even if no geocoded places exist.
All geocoded 'places' should show, but that means that they should all have defined longitude and lattitude stored with the 'place'. Have you included these details yet? Does the Google map link on the individual places?
It's a nice feature, but takes a lot of work to geocode all places.