I had precisely the same problem. My mast bearing balls kept falling out. I had to buy a new one because 1 of the balls went missing and the mast bearing needs all of them.
Let me try to walk you thru the fix.
The mast bearing has a top piece and a bottom piece - they are interchangeable, and of course all the nylon balls. The top and bottom parts of the mast bearing make a sandwich with the aluminum forward X-bar in the middle.
What has happened is that the sandwich is too thick which allows the gap in the mast bearing to be too wide which allows the balls to fall out.
Disassemble the mast bearing, look for metal burrs and high spots on the metal -Xbrace (top and bottom) and sand them all down. Sand the plastic area on the top and bottom halves of the mast bearing that mate against the x-brace. Area sanded is where the six holes are drilled. If yours are like mine where every hole is drilled will be a high spot.
Remember the goal is to make a thinner sandwich. Once all sanded - reassemble, keeping the notch cut for mast lock tab oriented correctly. Make sure you have all the balls - they should completly fill the entire diameter of the bearing - no spaces.
Tighten the screws up quite a bit and then check to see if your balls still fall out.
I have 2 Islands. On a very early model with original anodized akas & x-braces the clam shell mast bearing was 1/32" tighter than the Island with "e" coated akas and x-braces.
So 1/32" tighter should be your approximate goal.
My Thread over at KFS:
PS Many time it is the users who first find the problem and the fix not Hobie. Hobie does listen and many times adopts the fix or incoporate the suggestions into its production.