You should be able to get a testing unit that you plug in to your powerpoint, that you then plug the heating element in to, that tests the 'actual' power being used (I have one, and they are readily available in Australia). I have found that most of the cheap elements draw less power than they claim (note: mostly 240V in Australia).
I've used a Kill-A-Watt meter alongside my 2kW HotRod (same element as here) to check the voltage on the circuit* - at my last rental apartment, that voltage could drop as low as 108V under the load of the element, while registering ~115V under no load. At 108V, the output of the element is only 81% of what it would be at 120V - 1.65 kW.
The voltage can be lower than usual either because of the resistance of the circuit, as I saw, or also possibly because the power into the house is a bit lower than usual due to excessive load in the neighborhood (particularly on a hot day just before dinner time with ACs and electric ovens running). Element powers are specified at the maximum voltage, even though you might not have that coming from the wall.
The best option short of buying an extra element is then is to try another circuit or outlet, wait, or just possibly you could use a 20A variac to boost the voltage back up to the nominal 120V. I'm personally not too sure about operating a variac at that kind of load though, particularly a cheap one.
* Use the standard Kill-A-Watt with another outlet on the same circuit to check the voltage - it's not built for 20A, so plugging a 2kW element in through it causes it to heat up - I ran it for about 5 minutes like that, but I wouldn't recommend it. A multimeter is also fine for testing voltage, provided you can connect it to an outlet safely. Or you could build a volt and ammeter into your control panel.