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Here are some potential reasons why your circle overlays may not be appearing on the MKMapView:

1. Incorrect coordinate system: Make sure the coordinate system of your circle overlay matches the coordinate system of the MKMapView. The standard coordinate system used by MKMapView is WGS 84.

2. Incorrect overlay rendering: Check that you have correctly implemented the MKOverlay protocol for your overlay. This includes implementing the required methods such as coordinate and boundingMapRect.

3. Incorrect map view settings: Ensure that the map view settings are set up correctly, including the zoom level and the visibility of overlays.

4. Overlapping overlays: Check if your circle overlays are overlaying over each other that could be the reason for the circles not being visible. Try to set the alpha value of the circle overlay to a higher value.

5. Incorrect zoom level: If the zoom level of the map is too far out, the circle overlay may appear too small to be visible. Try zooming in on the map to see if the circles become more visible.

6. Syntax error: A syntax error in your code could also cause your circle overlays not to be displayed. Check your code for any syntax errors that may be causing the issue.

7. Not adding overlay to the map view: Make sure that you are adding the circle overlay to the map view by calling addOverlay() method where required.