- I started the plank work, but my dog is very shy&sensitive, a kind of like your Lo, and will slow down completely when I say "no" when she jumps at the end of a plank.
Oh, no, my Lo is lion brave in comparison to Bu... She had many traumas as the puppy, but she never shuts down like you describe - it's not Lo-like at all, when Lo has a feeling something is not quite right, she only gets louder and more hysterical, but will never shut down: more you frustrate her, more energy she'll be putting into it. What you describe is very Bu-like.
First thing you should do is to stop saying "no". I don't know how soft or not you say it, but I use very soft and almost happy "ups" - it shouldn't sound like "no, don't do that", but like "what a great joke, but you're still not getting a reward for that". HOWEVER, with extremely soft, sensitive, Bu-like dogs, even that is too much. So what I did when contact was not as good as I wanted was to simply send her around me and back to a plank, with a happy excited voice, but no click&no reward. Also, when starting with normal dog-walk, I just ignored the jumps, didn't stop her but continue a sequence so that it took me back to dog-walk eventually and when it was good, went absolutely frantic about how great she is. The funny thing with Bu is that even if you click& reward, she will sometimes not believe you she did good enough and starts to worry, so you really need to act as the happiest person in the world for like 10 minutes before she believes it. I was never so tired as I am after every training with her, I sure do miss La's attitude: "it's your fault, we all know I'm perfect". If your dog is a worrier, make sure you don't give him any reason to worry!
With Bu-like dogs, you can't afford too many mistakes + you can't afford marking them, so
1. don't say anything, stay happy, send her back happily
2. heave it even less as you do: one by one centimetre if it's needed for her to succeed
At this stage, you shouldn't be getting much jumping anyway, your plank should be so low that it should still be natural for her to just run. If positioning of the legs is not perfect, don't worry about it, reward everything that is not actual jump (meaning prolonging a stride at the end or making it higher). You will work on positioning of legs later.