We can try a suggestion that I gave another member who failed to report back if it worked or not:
"Ok, then I am wondering if the contact below the trackball assembly (on the circuit board) has lost it's "Spring effect". (There is not a spring but the contact shaped like a copper colored spoon) This contact floats above the circuit board and when you depress the trackball it forces it down to perform an "enter". You might want to remove the trackball assembly and see if it is almost flat against the circuit board. If so remove the battery and you can try gently bending it back up. this should also result in your trackball feeling "springier"...
Good luck and only do this at your own risk. Make sure the battery is out!"