You need to check the resistance of the Element... and check that the Thermostat is actually switching the power through, when it gets to temperature. Do a continuity test from the P & N, to the Element. usually the Phase comes in to the Thermostat, then through to the Contacts, to the "Hot" side, of the Heating Element, & back from the "Other"s side, the "Cold" side of the Element, to neutral. When thermostat is Operated, ON, When it gets to selected temperature it switches OFF, you should have a Resistance, measurable from P & N at mains lead in.
You can figure this resistance out by Dividing the Wattage of unit, by the Voltage this gets the Current, Now we divide that Current, into the Voltage and we get the "Resistance" of the "Load" or Element. This is what you should "See" with your OHMS Meter when you look "In" to the circuit. +/-10% is OK