When I want to publish my diagram that I published before, it is not visible in the model overview in the menu for publishing.
If your model already has a blue globe icon in the explorer view, the latest revision of your model had been published. Because it is the most recent version of your model, you do not need to publish it again. Therefore it will not show up in the menu for publishing.
KBA , BPI-SIG-PM-EXP , Explorer for SAP Signavio Process Manager , How To