Not all airports will display while using Synthetic Vision. This is due to there not being geo-referenced runway information for some airports.
The easiest test to know if an airport should show is to add that airport to your route and then make sure EXTENDED CENTERLINES is turned ON under MORE > SETTINGS. Now, if you see extended centerlines emanating from that particular airport, it should show a runway within Synthetic Vision. If you do not see the extended centerlines, the airport will not show within Synthetic Vision. See image 1.
Image 1: Airport with no extended centerlines showing means there is no data to allow the runway to be displayed within Synthetic Vision.
Also, a runway may be obscured by terrain. You may be seeing extended centerlines and the airport marker, but terrain that is in the way will prevent the runway from showing. See image 2.
Image 2: Runway will eventually display, but for now is not showing due to terrain obscuration.
If an airport has extended centerlines available and no terrain is blocking the view, then the runway will be displayed within Synthetic Vision. See image 3.
Image 3: Runway has extended centerlines, which means it will have the data to show the runway within Synthetic Vision.