Cannot select text in textual analysis

Sometimes, on the textual analysis screen, I can’t select the text in the text window. I can initially select text, and right click it to assign it to an actor or whatever, but if I go change the name of one of the actors or some other task, and then go back to select some more text, it won’t let me select any more text. I have to close the application and reopen it.

Any ideas what’s causing this?