[QuickSilver]

What are you using for the treat? I've noticed that Thor is way more attentive if he really wants what I'm offering. If you use something super-enticing, her desire may overcome her fear.

I would also say 30 minutes of unsuccessful training is a long time. I would try maybe a dozen times, and then switch to something else. As you said, maybe just teach her to lie down first. Does she know any other tricks? The first one is a big one, since that teaches the dog that she'll get the treat if she figures out what you want. Try to always end on a positive note.

Also, are you sure you're using the right hand gesture? The treat should be at nose level and then sloooowly move the treat forward, just a little bit over her nose. If she's jumping at it, it's probably too high.

Some dogs it just takes a while. If you can't get her to sit, reward her her if her butt moves even a little bit towards the floor, and work from there.

One final thought - do you know the hand gesture for the sit command? That mimics an action that should make the dog naturally sit. Offer the treat between your fingers, palm up. Then curl your fingers up and towards you, as if beckoning to someone. This should naturally cause the dog to raise its nose, pushing the butt down.