This is because of your window component options. Go to edit window in PlusSpec and select your window.
In the component options you will see an option that says ‘Max. Window Height’. This is the most important option for highlights.
A highlight will only be drawn in if your window height is larger than your ‘Max. Window Height’ in your component options. I cant read what your ‘Max. Window Height’ is in your component options but from what I can make out it is larger than your window height in the window tool. I can see your window height is 1800mm so for you to get a highlight you need your ‘Max. Window Height’ to be less than 1800. EG for a 200mm highlight you will need to make your ‘Max. Window Height’ to 1600.