Well, rein contact is actually set by the horse, to a certain degree. Waht I mean is, you put some contact on, and the horse has the choice to either ignore it, lean harder on it, or react to it by coming off of it. So, he/she is setting the contact.
You decide waht you want, and don't give up til the horse gives you THAT. So , if you want the horse to flex a bit, you have contact for that, and when flexing happens, the horse has come off the contact. You tell the horse, "yep, that's what I wanted" by looseing the contact. Eventually, she will learn the "window" of freedom. If she keeps her head between this point and that, then you take no contact.
But even better for bringing her back into showing would be to just ride her forwaard a bit and not worry too much about the headset. She just needs to move forward and loosen up and build some muscle. What if you just spent a few days not really taking much contact, unless you needed to, and then released immeditately (practice sliding your hand up and down the rope reins to shorten and lengthen the contact and be sure you dont' lean forward when you slide your hands down). Get her moving out for a bit. She seems to have no umph right now, so putting emphasis on shutting down her front end will discourage any push from behind, since she has so little right now to begin with.
However, keep in mind, I dont' ride WP. So, my POV is maybe not in line with the WP trainers approach it.