canEdit problem in 1.8

canEdit returns false in some cases, I dont understand why. I use plugins from other developers and this promblem returns in some cases. e.g.
Even in the original 'messages' module of elgg is a bug at me. Using the 'Mark read' button, and then nothing happens. I debuged the code and I recognize that the canEdit() methode returns fals.
I don't understand why, is this a bug, or what?

Thanks for the help...