In-fridge icemaker makes hollow ice cubes. How to fix?
I eventually fixed the problem, and it turned out to be insufficient water flow to the icemaker. 

The ice maker was being fed via a clamp-on saddle valve. I closed that off, disconnected it, and soldered in a 1/4" brass line with a 'proper' shut off valve. That provided plenty of flow rate, and since then the icemaker has been working flawlessly.