Diagnosis
The machine is freezing water, but it's not getting enough water into each mold cavity. This is usually because the water inlet is partially clogged or the water pump is getting weak.
How to Fix It
You need to run a cleaning cycle. 1) Unplug the machine. 2) Mix a solution of equal parts white vinegar and water in the reservoir. 3) Plug it back in and press the Power and Ice Size buttons together for 3 seconds to start a clean cycle. Let it run the full 15 minutes so the vinegar can break up any mineral deposits in the lines. After, dump the vinegar water, rinse the reservoir well with fresh water, and run a cycle or two of plain water to flush the system.
If That Didn't Work
Check your water source. If you're using distilled or filtered water, try switching to regular tap water for a batch. These machines sometimes need the minerals in tap water to help the freezing process and sensor detection.
Heads Up
Don't use chemical descalers or bleach. Stick with vinegar. Stronger chemicals can damage the plastic and leave a residue that affects ice taste.