Its probably filthy. Its probably seized up because of dust & sweat that builds up over time. take the ball out give it a good wipe, look inside and get a small brush and give it a brush out. How long ago was it you did this? Should be cleaned every 2 - 3 months.
Thanks, Jem. Sadly, mousy is of the eunoch variety ( No ba**s !) Still, he may quite like some sort of service if one is available. Otherwise he may have to be retired.