While creating a communication uml diagram I've found that the properties tab won't reflect the communication message properties when one is selected in the canvas. The properties tab keeps showing the communication channel properties. This makes impossible to set some properties of the communication meesage like the invoked method and so.
You'll find a screenshot attached with an example of this issue.
I reproduced your issue. It seems that the CommunicationMessage selection in a diagram is not right.
You can post an issue here
to follow its resolution.
In the meantime, you can still make the CommunicationMessage properties appear when selecting it in the model browser as shown below.