Over time, rubber drive belts lose their elasticity, become worn with age and usage and so lose their grip. If your belt has been unchecked for a period of time, you may also find that it has disintegrated and has left a black gooey residue - care should be taken when removing these old belts so as not to cover sensitive parts of the drive train or bearings in any residue. 

Replace your old drive belt with one that has recently been produced, which provides good grip and tension. Prior to installation ensure that the belt path is clean.