DIAGNOSIS The machine isn't getting enough water into each cube mold during the fill cycle. This is almost always a water supply issue, not a refrigeration problem.
FIX Check your water source first. If you're using filtered or softened water, try switching to plain tap water for a few cycles. The pump in these is designed for a certain water pressure/mineral content. Next, ensure the water reservoir is above the 'MIN' line but not overfilled past 'MAX'. An overfull reservoir can create a siphon effect that messes with the pump's draw.
IF THAT DIDN'T WORK The water inlet valve screen might be clogged. Unplug, remove the reservoir, and look at the small circular inlet on the back of the machine where the reservoir connects. Use a toothpick or soft brush to gently clear any visible sediment from the tiny holes.
HEADS UP Ambient room temperature matters. If the room is very hot (above 90°F), the refrigeration cycle can't freeze the water fast enough before the cycle ejects, leading to weak cubes. Try moving it to a cooler spot.