Actualy its not the trackball, its whats under the trackball that makes a physical click feeling.
Theres a small piece of metal that is bent in the middle. Sort of like looking at half a bubble. This small piece of metal comes into contact with the lower part and causes the circuit to be completed. Basically this piece has been pressed down so many times its loosing its shape. Its held on by a piece of tape and can be replaced if you have another BB. LOL i repair them so in my office i have a few dead BB laying around for parts, but i know what your experiencing. THis can also be due to debris or in some cases liquid that will get under the tape and causes it to stick, however not enough to limit the circuit from being opened or closed.
Hope this helps.