It's fixed mine a couple of times. Make sure the water is warm but not necessarily too hot, keep the crown turning whilst running water over it.. the last time I did it I could feel it suddenly become freer and 15 seconds (as recommended) is actually quite a while for a rinse.
If this doesn't sort it then one of the other posters said that they used flat, unwaxed, dental tape/floss to clean underneath the crown. This might be worth a shot but it's not Apple recommended of course and as the saying goes 'If you break it you get to keep both halves'
If this doesn't sort it then one of the other posters said that they used flat, unwaxed, dental tape/floss to clean underneath the crown. This might be worth a shot but it's not Apple recommended of course and as the saying goes 'If you break it you get to keep both halves'