MKMapView not alway call the delegate to show the annotation

Question

I have a mkMapView with one annotation. The first time every thing is OK. The application call the delegate - (MKAnnotationView *)mapView:(MKMapView *)theMapView viewForAnnotation:(id )annotation {...} and my annotation is working fine.

When I need to change to a new map location (with a new annotation) the old one is release the new annotation is created but sometimes the new annotation appears (the delegate is invoked) and sometimes not.

To show the annotation I have to Pinch the map and usually the annotation appears (not always anyway...)

There is a trick to force the delegate to call the delegate or to refresh my annotation ?

Thanks for help !

Dario