Summary
A BPMN element must be part of a Lane.
Details
When a BPMN Process contains lanes, all its BPMN elements (tasks, events…) must be split between these lanes. In this case, the "orphan" element can’t be displayed in diagrams.
Tips
Select the BPMN element that is not part of any lane and either move it into a lane belonging to the same BPMN Process or delete it.