DIAGNOSIS This is a classic sign of scaling. Mineral deposits from your water have built up on the evaporator (the freezing plate) and insulate it, making heat transfer sluggish. It's like trying to freeze something through a blanket.
FIX You need to descale. Use the cleaning cycle method I described earlier (hold "Ice Size" + "Power"), but with a descaling solution. If you don't have the commercial cleaner, a strong mix of 16oz water and 4oz distilled white vinegar works. Run the cleaning cycle, then let the solution sit inside the powered-off unit for an additional 30 minutes to really break down the scale. Then, run two or three clear-water cycles, discarding the ice each time, until you no longer smell or taste vinegar.
IF THAT DIDN'T WORK Consider your water source. If you have very hard water, scaling can happen quickly. After descaling, start using filtered or distilled water in the reservoir. It's a hassle, but it prevents the problem from returning so fast. Also, make sure the room isn't unusually hot; these portable units struggle in ambient temps above 80°F.
HEADS UP Don't use CLR or other aggressive chemical descalers unless specifically says you can. They can damage the food-grade plastics and seals inside the machine. Vinegar is safe and effective.