The centre, plastic "widget" is probably starting to break up. They all do, eventually.
Spare parts are no longer available.
AFAIK only one person has succeded in reproducing it. See here
http://forum.beoworld.org/forums/t/37981.aspx
In the meantime I suggest that you look carefully at your plastic "widget" and repair with araldite or similar before you loose the bits which it will inevitably break into.
B&O did once supply a small, flat ring magnet as a solution to the problem but also, alas, no longer available. You could try a local hardware store or the internet.