Sounds like he's put too much hardener in the mix. Don't use quick plug, just the regular stuff. Then, if the hole is fairly large plug half at a time. The larger the area, the hotter the mix becomes, thus setting up too quickly and cracking occurs.
I was reminded of that a few weeks ago, while plugging another ball. I had completely drilled out my thumb insert, so the hole was good sized. Then, forgetting about the above, I plugged the whole thing. Next day the cracking at the top of the newly plugged area. Drilled halfway out, re-plugged and it was fine.
It really has nothing to do with the particular ball, because I've had it happen to balls from all manufacturers. It's the method.
