The issue that you're seeing (that it only works the first time) is also the issue that I'm able to reproduce. As I said in the post above, the workaround - for those using the latest versions of Scratch Live - is to create the video associations using Video-SL (I believe there is a "Set Video" button, or something with similar wording, that will pop up when VSL is activated). I would recommend that you re-start Scratch Live once you've made all your associations though, as you don't want to be running VSL and MixEmergency at the same time during your performances.