The reason the Specialist to Network view does not appear highlighted is because the other four worksheets are being linked by the "Diagnosis" field, but the Specialist to Network view does not have a way to be linked. If you go to each of the worksheets, you will notice that each of the worksheets, except "Specialists- N/W", will contain the "Diagnosis" field somewhere in the view. As a test, add "Diagnosis" to the Marks card on the "Specialists- N/W" worksheet. The graph will appear a lot differently, but that is okay just for testing. Next, go to the "Sankey" dashboard and hover over once of the marks in the "Specialty" worksheet and the middle Specialist to Network view should be highlighted.
To resolve this issue, the Diagnosis field must be placed in the view (in the Specialists- N/W worksheet) and the table calculations may need to be adjusted to account for the additional dimension. It may also be necessary to adjust the order of the dimensions on the Marks Card.