Sometimes it happens that in the dialog for creating diagram (Add Diagram…, New Child Diagram|…) or changing the type of diagram (Change Type…) the list of diagram types listed under [Type] (eg UML Structural, UML Behavioral) is unexpectedly restricted or even empty.
Then the current perspective is probably the wrong one, or it was inadvertently changed.
The solution is simple: Click on the [Select from:] header and select either the corresponding perspective set or [Manage | Show all Perspectives] from the context menu that appears.
