Remove the TOP COVER of the dryer (2 screws at the rear of the Top Cover---slide cover back an inch or so---lift off)
On the drum---you'll see a BELT. Grasp the BELT---it should be *tight* against the drum.
If not---order a new DRUM BELT and MOTOR PULLEY.
The MOTOR PULLEYS were enlarged from 22mm to 27mm to prevent the BELT from slipping off the IDLER PULLEY if the drum is rotated-by-hand *counterclockwise*.
Samsung Dryer Drum Belt Part # 6602-001655