Ok I sussed out the problem. Teachers were using cut and paste to move a forum down the page nearer to the current week/section of a course (intead of using the inbuilt 'move' function of moodle). They say they feel more at ease with 'cut and paste'.
Thus the forum was an exact copy. And when they prepered a week in advance and made the (exact same) forum for that week hidden - all the (exact same) forums became unavailable for students to post to.
// Tony