The problem may be that you are trying to calibrate over an area that has house electrical line protectors underneath the sheet rock. These are flat galvanized steel plates positioned to prevent someone from drilling through the wires. The same thing happens with heavy gauge a/c duct work under the surface.
I can think of a few I've run into here are 3: 1) you are over a metal strip on the stub meant to protect wiring underneath. 2)there are no stubs, some construction materials are plastic, cardboard, styrofoam or simply not there defying good construction common sense. 3) duct work is behind wall. My neighbor is a master carpenter & does finishing work. We talk about crappy construction we see all the time. And without seeing your project it is impossible to get specific.
Bring it to a dealer where they can comm with your phone during same procedures. Warning, you may be looking at an irreparable problem. It may have moisture damage from going in & out of doors.
In addition to the preceding answers there is the possibility that the GFCI has simply worn out or become defective under load. Check the appliance by running it on an other GFCI. If it works fine...replace the GFCI.
