VoiceOver image description feature not working as expected on mac

Hello everyone,
Ever since I upgraded to MacOS 10.13, I've heard a lot about the VoiceOver image description feature. But when I personally tried it, it doesn't seem to work, and I hear this message very often, saying, "This item has no description." Am I using the keystroke in the wrong place? Or is it just another of those accessibility bugs? I've tried using it on the google images search results page in chrome, and to try reading images in PDF's, but it just doesn't seem to work as expected. I'd like to know how I could possibly solve this tricky issue.