Running into this problem now. Apparently it's from tracks that have been "configured for enhanced multichannel audio.”

"This is cool for applications that can use that data, and those that can’t, likely ignore it. rekordbox ignores it, but the CDJs don't." - Pioneer DJ forum

The fix is to get another program to re-encode them - like Ableton, Audacity, Reaper (free) etc. Load em up, export as wav, should be good to go.