Hi, James,
It is definitely possible to import DV tapes into iMovie. iMovie supports most tape based camcorders. Here's a help link with step-by-step instructions that you might find helpful to use as a troubleshooting procedure:
http://help.apple.com/imovie/mac/10.1/#/mov6af9c94c1
Several posters on this forum have reported that DV imports have imported with no sound. It seems to be related to either iMovie 10.1.8 or High Sierra, or a combination of the two. Some users have found that the DV tape will import with sound if imported into iMovie 9.
Apple recommends to import photos and videos into the Photos app for use in iMovie, rather than directly into iMovie. You can also use the Image Capture app on your Mac to import to a folder on your desktop. Another thing you can try is starting the camera playing once connected, to see if that causes it to be recognized.
How are you sure that it is not the cable? Sometimes there are problems with adapters as well. Have you tried changing out the cable?
-- Rich