« Good...but with a major problem »

It's the first time I use a USB mixer. I've been using the SCS 4DJ for two months. I sent back the first one after a month because the buttons would block and there was no way to stop it. I love the fact that it can read directly from a USB key or a HDD without the need of a PC. The second one works fine, except for the decks (like the first one): Every time I try to move along the song I get an error message: "The song requires a function that hasn't been enabled," and the song disappears from my playlist. Since this is the second mixer with the same problem, I wonder whether it's a known defect or it's just me who doesn't know how to use it??? I wouldn't buy it again unless I find a solution to this issue.