Due to lack of regular tuning slide lubrication or fast climate changes, horn's tuning slides might get stuck. The tip presented on this video can solve most of the situations. If the slide doesn't come out at the first try, please don't force or you might get a broken joint. For serious situations do not try to be super man... take your instrument to a specialized repair technician that will easily solve your problem.
English subtitles available, just press the "cc" button bellow the video!