Assuming it is mains powered, it could be as simple as a broken wire where the cable enters the trimmer that makes contact when turned in the correct way. Try wiggling the lead at the body as well as at the socket end and see if it does the same thing. If it does replace the lead or if not comfortable with doing it, have an electrician replace it for you. If not, then you could be right about the brushes but I am not familiar with the model at all. Some photos are always a big help.
