Please ensure that you are using the latest version of Lightroom CC. Also, does this only happens when you import a DNG image from iPhone?
Thanks Akash. Yes, I have the latest version of CC. The only device I have for DNG files is my iPhone 7 Plus. My Nikon D7100 shoots NEF.
I've attached two screenshots that show the preview for the DNG files in the multiple image view and what happens with I double click on one of the DNG files. I can see the images fine when I have them full screen, but not when they are in the multiple image view.
If the files are DNG you need to choose “Copy”
Only use copy as DNG when converting from NEF or other raw file types.
I changed to COPY and the previews are still "Preview unavailable for this file".
I’m not sure why. It may be easiest to select all and import the whole batch and wait for the Library previews to get generated. Then make your pics and rejects.