Virtual Connector - Don't appear in context menu

[Sunshine]

I've quite a complex diagram and need to use the virtual connectors to reduce all the connector crossovers.
From the manual
https://sparxsystems.com/enterprise_architect_user_guide/17.1/modeling_fundamentals/virtual_routing.html
It says On diagram | Right-click on connector | Virtualize Connector End | Target (or Source). However I don't see that in the context menu.
I've discovered that the virtualize connect end appears on some connector on some types of diagrams but not all. I've even discovered that I have to diagrams the same type with the same element types and relationships and it appears on one context menu in one diagram but not both. Really puzzling.