i have a new ball cock in toilet but it keeps coming of, is it faulty, the ?

i have a new ball cock in toilet but it keeps coming of, is it faulty, the plumber said pressure is making it come off, maybe a sege of water, but today it is off again, was going to try and fix it my self
i can see where it fits in, but not sure which way it goes it does not look like other ball cocks..its not a ball more of an holow ovel.

Ruth Todhunter
April 2009