My Hotpoint Aquarius tumble drier TDC32 model, clothes all ball up?

My tumble drier appears to be working - heating up etc, but every load, the clothes all ball up into a big bundle and dont dry properly. Although the dryer sounds like it is stopping at intervals and one would presume reversing the tumble, is it likely that it is not reversing? this is the only explaination we can think of. If that is the case, how would we rectify it? thanks