Cannot modify Message Properties

[ShaneC]

I have created a UML sequence diagram and have various Lifelines and Messages.  The first 5 messages I created can be modified by double clicking on them but any new ones I create cannot be modified.  When I double click on them all the fields in the screen are empty.  If I enter a message it replaces the old one though so it does understand I am trying to modify that message.

Any thoughts?