when notes files changed on disk (in my case, as a result of syncing between two computers I use interchangably), the xfce4-notes that is running in the taskbar does not notice the change (doing a 'touch' doesn't help) and continues to display older text for the note. Having the program check/store modification times before displaying a note would be one solution; in my case, having a signal that could be raised that would result in xfce4-notes re-reading its notes files would also do the job. Fedora 20x64, if it's relevant.
I have exactly the same problem. I found very useful to synchronize my notes across different computers (so I can remind them regardless where I am or what I'm doing).
version 1.8 uses glib.FileMonitor, a button is displayed in the notes in the top right corner when the file has been modified externally.