Missing operations in sequence diagram

[Maarten]

If you use classes or interfaces as lifelines on sequence diagrams you can normally use their operations as messages. From time to time it seems impossible to access these operations. E.g. suppose you have an interface that you drag onto a sequence diagram to use as a lifeline. If you draw a message towards this interface, you can select an operation that was defined on this interface from the dropdownlist.

However sometimes this dropdownlist remains empty although several operations are defined for the used interface. When you push the operations button you normally go to the operation screen of the interface where you can add or modify operations. When the dropdownlist remains empty pushing on this operations button has no effect either.

Does anyone have an explanantion for this behaviour?

It happens both when you add a message to a lifeline that was already drawn on the sequence diagram or when you add a new lifeline. Closing Enterprise Architect and restarting it usually resolves the problem. Nevertheless I do not really like this solution and would like to know the reason for this behaviour. Any reaction is appreciated.