As has been said this is a well known issue. If it's under warranty Sofabaton should replace it, they did with me. Contact cleaner will solve it for a while but I ended up with the new one doing it as well, and at the point of spraying it every few days and out of warranty. The only long term solution is to take it apart, strip the ball down and clean it properly. Having now done this I haven't needed to spray for a few months now. Dirt/grease gets in between the rubber outside and plastic inside of the ball and slips.