Help with Heatmap not working

Firstly, I am new to this game and just experimenting with sandbox.
I have a dive coaster the layout of which I have significantly changed. When I do a test, the heat map seems to suggest that it is still testing the old layout.
See attached photo.
When I do a test on one of my other coasters, the "Live" heatmap follows the coaster train. It doesn't do that on the dive coaster.
Can anyone help?.
 
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