By default, the date on a file downloaded by youtube-dl is the date the video was originally uploaded to youtube. As a result, items will always be cleared from the cache because they appear to be years old - even though they were just downloaded. Adding the directive "--no-mtime" will make the date on files the date the file was downloaded.
See this issue on youtube-dl: https://github.com/rg3/youtube-dl/issues/4667
