I'm with Don on this one. If you have 240V measured across the heating
element and the heating element is good per resistance check, then it
has to heat. At least at that point in time. Keep in mind some
heaters are on a timer system controlled by the electric meter, where
power is only provided off peak at a reduced rate.

I also agree with the advice that it was a mistake to replace the
working element and 2 thermostats. In my experience, the element
failure doesn't appear to depend much on length of service. And I've
never had a thermostat go.