My first instinct would be to check if you have added any demand for fuels into your model. This may seem obvious, but it's unusual that none of the fuels are connected to demand.

Please note, however, that we are phasing out the Diagram View in favor of Sankey Diagrams, and so we are no longer able to provide support for the Diagram View.

http://www.energycommunity.org/WebHelpPro/Views/Sankey_Diagrams.htm

Please make sure you are using the latest version of LEAP before trying to view its Sankey Diagrams. Apologies for any inconvenience that this may cause for you,

Taylor