You need to meet Meg
.
When I first got her, she wouldn't even walk over a flat board on the ground. Anything she didn't know (and having spent most of her life as a tied, outdoor dog, she didn't "know" much, including stairs and what a house looked like from the inside) was terrifying. Shut down? If something scared her in agility class, she was under a table and that was it for the night. I had a few classes where I wound up running my trainer's dog, in an effort to get her jealous and interested again. Not only could I not used verbal or physical corrections with her, if she tried something and didn't get rewarded two times in a row, she was done and would shut down and refuse to do anything.
Agility, and even more importantly, THE RIGHT TRAINER, absolutely turned her life around. Not only does agility not scare her any more, but she has gained a remarkable ability to bounce back when things in the real world scare her. I rarely see her even nervous any more, and haven't seen that extreme shut-down behavior in at least a year. She's a new dog, and went from "maybe we'll do a session or two to build her confidence" to being very close to getting into the highest level of competition in USDAA.
You have to be careful. You have to go incredibly slow, and get out the A-Game rewards for ANY attempt at bravery. I would look for the littlest things - just having her tail lift from tucked in to having air between it and her legs - and reward that. If anything ever started to bother Meg, or if she wasn't getting what I was asking, we'd move on to something simple for a few reps, so her reward percentage would go up and her confidence wouldn't suffer.
Good luck! It can be so frustrating to work with these softies, but oh, boy is it ever rewarding when things start to come together! Does that look like a scared little girl to you?