Diagnosis
The heating element is degrading and failing to reach its set temperature, or the thermostat is miscalibrated and cutting power too soon.
How to Fix It
Test and isolate the problem.
1. Set the iron to its highest temperature and let it heat for a full 5 minutes. Carefully (and quickly) tap a drop of water on the plate. It should sizzle and evaporate almost instantly. If it just sits and boils slowly, it's not hot enough.
2. Check the plates for perfect alignment (see the gap fix). A gap means less contact and less heat transfer.
3. Clean the plates thoroughly. A thick layer of residue acts as a heat insulator.
If That Didn't Work
The heating element is wearing out. Like a light bulb filament, they degrade over time with use. The iron is approaching the end of its functional lifespan.
Heads Up
Trying to compensate by leaving the iron on longer or on the highest setting permanently stresses the failing components and is a fire risk. If it's not heating properly, replace it.