Ripleygirl I think there are three possibilities for your stand tall problem, I've listed them here with some ideas of how I'd tackle them - but I'm no expert
a) Most likely is she is depending on being able to see you so it is probably just a matter of proofing her stand tall - so she learns she can do what you say when she can't see you. Have you tried with her sitting in front of you but facing away? Then ask for a sit up and beg, then when she is comfortable with that a stand tall. It helped Zac because he was then not looking at me but I could see what help he needed. The only thing is that he can knock me backwards when he stands tall in front of me however Zac found working behind me much easier after a couple of sessions doing this. Then work on sit back to back, then sit up, then stand tall.
b) Rather than hearing your verbal cue she is taking a body cue from you that tells her to stand tall and you are unconscious of giving it to her. I don't think this is likely because the cue would have to work whether she was looking at your front or your back but it is a possibility. It is a matter of keeping still and giving her a verbal cue and then luring her a second later if she doesn't respond until she gets that the verbal cue means the trick (You'll know that already)
c) Also unlikely but possible is that she is depending on you for support - I don't know how secure she is with stand tall but even if she isn't actually resting on you she is may be like when you have first learnt to swim and you stay really close to the side of the pool in case you need to grab the rail! I'd just practice stand tall in different positions and click as soon as she rises up without asking for duration. Zac used to like facing me so he could stab out a paw (usually muddy) for support if he felt unsteady but he has got more comfortable with it now he just leans backwards into me and I end up staggering.