I’m also having a tough time envisioning exactly what’s going on with this girl’s starts, but I suspect it has something to do with integrating a specific new movement into a well-coordinated sequence. With less talented or less athletic individuals, if I tell them to do “x”, they tend to do “x” and not much else.
What to do? Well, I like the suggestion of having a “just f*** it and run” drill, where the athlete is focused simply on achieving a feel or a rhythm or an outcome, rather than a specific movement. The goal is to see the movement-specific cue carry over in a more fluid or functional fashion.
If things are still terrible, they’re usually terrible in a slightly different way, and you’ll get an idea of what needs tweaking when they’re actually running, instead of overemphasizing a particular element of their start.