Could be any one of a number of reasons. May be only agitating one way causing clothes to tangle, may only be spinning the agitator and not the spin bowl as well, could be worn/weak suspension springs. Impossible for someone to say exactly when we can't see what is happening. Suggest you call out a service company.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix washing machines.