It might reset itself. My M6 did the same thing but the problem worsened and eventually I had to get it fixed -- a little gear or something had become disengaged. My M7 has sometimes done the same thing but has subsequently reset itself. If it bothers you sufficiently to get fixed I think it is a small and inexpensive job. Someone else may be able to tell you how you can do a DIY adjustment, however, but I'm afraid I don't know.