Hi Bonefish - I commend you for sticking it out here. It is hard to lay out what you've done and get a lot of 'critique'.
Your pup is only 6 months old? If so - that is YOUNG. Perhaps you've put yourself on a 'set in stone' timeline of training? If so, I suggest you trash that idea and move to a more fluid timeline.
Something else to think about. Imagine that, from your dog's perspective she sees a huge fire and bed of coals between her and the pile of bumpers - maybe even two or three beds of coals and fires that she has to cross.... Not only that but she also believes this fire and bed of coals will get bigger and hotter each time you give the 'back' command. She does not want to put herself in a dangerous situation (in her eyes, not yours!) and so she refuses to go.
Compare that to a dog who's been properly conditioned to respond to the collar pressure. They look out and see a little smoke. They understand that the pressure may (or may not happen), but they are not so fearful of it that they outright refuse to take a command. Conditioning is key. They know they will succeed every single time because they were shown how to negotiate the 'fire' and put it out.
I remember talking to a FT'er some time ago - asked him how a recent trial went (this is an FC AFC dog) ... He said they went out in the second series. She no goed on the blind. Dogs CAN see so much danger out in the field that they WILL or MAY no go. In their mind, the punishment at the line is better than what they think is going to happen out in the field. That's how I see it.