GFI's go bad in two ways. I had one GFI that never tripped it was great, found out it was bad, got shocked, and was not working. GFI wear out and will trip real easy, found this out on a construction job, after electrician changed it was not as bad. Have you guys checked your extension cords to make sure they have the right polarity. They make a plug that will plug in to a cord or socket and it lights up a certain way if everything is right. Check the plug that you plug into first, if it is not right the cord will not be right. I have found new cords that had a lose/open ground.
01 Chevy 2500HD 8.1