The problem is not in the trackball, but in the contact on the motherboard (at least in my unit it was). I had an old pearl laying around so I cannibalized it for parts. It's a pretty simple fix but it requires patience, a q-tip, some rubbing alcohol, and a pair of pointed tweezers.
When you remove your trackball you see a raised square that is covered with a piece of copper colored tape (of the same size). That tape secures the convex contact that is responsible for making the "click" sound and providing the feedback when the trackball is pressed. The problem is that sometimes these become worn out and lose their elasticity. Solution? Replace the contact.
Like I said before, I had to remove this part from an old phone I had laying around. I couldn't find this particular part anywhere on the net. Test to make sure your spare part makes the click sound before you remove it and put it in your new(er) phone. Here's how it works:
Take the batteries out of both phones. Remove both trackballs and use a q-tip and some alcohol to make sure the parts are clean. Remove the contact in the scrap phone by taking a pair of pointed tweezers and working the edge until you get a corner up enough to grab hold and peel away from the motherboard (note the orientation of the part relative to the rest of the phone before removing). Be careful not to tear the tape or damage the part when removing. The contact (small circular convex metal piece in the center) should remain intact with the tape.
Assuming you have a usable replacement, remove the damaged one from the other unit in the same manner and set aside. Using your tweezers, position the working contact in the exact same position as the old. Make sure that the edges are flush and square on the motherboard. When you think it looks right, take the dry side of the q-tip and press all around the edges to help the residual glue to adhere.
Replace the trackball, retaining clip, and chrome o-ring and give it a press. It should feel exactly how you remember. Replace the battery, turn the phone on and cross your fingers
Mine worked perfectly - hopefully yours turns out the same.
PS. This is the same part in both the pearl and the curve, no worries.