Please check if the column 'Who?' is locked for edit.
Some diagrams contain non-linear processes, for example, decisions or parallel workflows (gateways) or multilevel roles. You can open these diagrams in QuickModel to edit and complete those diagrams but it is not possible to edit them structurally, for example, to add or remove tasks or roles. These buttons appear grayed. Columns that are not editable are marked with a star symbol.
You can find more information about QuickModel here.