It seems that sometimes address lookup / reverse geocoding doesn't work reliably or cannot work because I had no internet connection when I imported photos. In these cases, no location information is looked up and it also seems this doesn't happen again later. Is there any way to trigger this process again manually?
Please refer the discussion: What Reverse Geocoding Is and How to Do It | Jim's Pics
You can do this manually by selecting every image in Map module and select the location on the map.
Selecting the location on the map is not what I want. The images have GPS coordinates attached. But sometimes the location lookup doesn't seem to work. So the location fields (reverse geocoding) are not set in the meta data.