Two causes the stylus is worn, or there is not enough pressure being applied to the stylus. Most cartridges track at 2-3 grams. The weight at the end of the arm should be set around this figure and slowly moved higher till it tracks without sliding and without creating too much rumble in the sound. If you have to take the weight up high, then that's a good sign the stylus has gone. Or that the arm itself has a fault on it somewhere (when tried with a new stylus installed).