They have collected too much dirt from the dust that lives under the gondola shelving. A few drops of the provided oil on the 3 balls bearings leave FOURMI skate looking like new and ensures its longevity. If the ball bearings are really clogged, oil them and leave your FOURMI skate upside down overnight. You will then be able to remove the dirt with a clean cloth and your skates will be ready again for moving gondola shelving.