Sometimes a reference cannot be resolved because necessary model is not imported or owner module is not configured properly (the dependency from some other module is missing).
If this is the case then the model/module nodes are highlighted in project tree.
It is also often handy to find references which was brocken as result of such improper configuration.
One way is to open the model properties dialog and press "show missing refrences" button
Or you can use model's popup menu
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