Subliminal messages can work, but not for everyone. If you don't find success with one method, you can always move on to another one. I wouldn't say the song is causing the drop in your dream vividness; that's most likely just a coincidence. I'd combine it with other forms of MILD, such as writing out that you'll have a lucid dream, or saying it. You can always try WILD, too, since you've gotten SP a few times now.