Why isn't Downloadhelper working with Youtube?

I have been using the Downloadhelper add-on for years on my laptop with no problem. I noticed about three days ago it was no longer working with any Youtube videos. The rotating icon is still working, and when I click on it, it still gives me the file type choices for downloading. However, after I click my choice, the green arrow shows the downloading process, but comes up '0 bytes' for every type of video. I am not having problems downloading videos from other sites that I've used before, just Youtube. Also, I am able to use Downloadhelper with Youtube on my other computer without a problem. Can you please help? Thanks.