Hi,

Adobe DNG Converter (and Lightroom, AFAIK) can create DNG file which can contain medium or full size preview (but not both in single file). Before extracting preview, you should know which size DNG contain.

To extract medium preview, use:
exiftool -b -PreviewImage FileName.dng>MediumPreview.jpg

To extract preview image from DNG which has embedded full preview, use:
exiftool -b -JpgFromRaw FileName.dng>FullPreview.jpg

So, for example, using -JpgFromRaw with dng which has embedded Medium size preview, will give you nothing.
I hope that will help you... Greetings,
Bogdan