Diagnosis
The temperature sensor is miscalibrated or faulty. The unit thinks it's at temperature, but it's actually much cooler.
How to Fix It
Perform a reset. Unplug the unit for 30 minutes. This can clear a temporary error in the control board. Plug it back in directly to a wall outlet. Select the highest heat setting (usually three blue lights). Allow a full 5-minute heat-up cycle before testing on a small section of dry hair.
If That Didn't Work
The thermistor (temperature sensor) attached to the heating element is likely out of spec. This is not a user-replaceable part. The entire heating chamber or base unit must be replaced.
Heads Up
Using on damp or overly conditioned hair will also cause curls to fall. But if the barrel isn't painfully hot to a quick touch (be careful) after 5 minutes on high, the hardware is defective.