Do you have access to a scope and a current probe?
Does your clamp on show any current on the EGC? (although they are not usually that reliable at GFCI trip currents)
If you are doing this I would start on a dumb receptacle so nuisance tripping is not shutting you down before you see the fault.
I have done this to prove refrigeration equipment had a real fault to people who say "they all do that".
