ESPECIALLY don't connect/disconnect a mic from your controller. restart the song once loaded into the venue. The last one is usually the key piece that's missed. The RB4 engine has issues locking onto video calibration on first load of venue, and even loading into subsequent songs within the playlist. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.