I have not been able to return to followup.

some notes to respond to questions.

This is a newly purchased plugin wood stove looking electric heater. Not sure, but I think 1500 watts. Two were actually purchased. The other one runs fine on its circuit. This one runs fine on the other circuit.
The circuit in question has always been a problem with tripping breakers when portable heaters were plugged in.
The receptacle being used is the first device on the circuit. It is about 30 feet of cable from the panel.

When I go back, hopefully this weekend. I will open the receptacle again and inspect the connections. I will move the breaker to a different slot. I will try the breaker from the circuit that holds, instead of the breaker easest to get to.