Kirk,
The dng files I'm referring to are the native raw format of the Leica camera I use. They are not converted from another format.
I have figured out that while I do not get a preview if I double-click on the file, even though I have set the double-click to open a the preview window. I can get a preview with ctrl-R. But that preview is not very useful because it shows "jaggies". Perhaps there is a setting for cache or preview size that would change that.