Can you post the log file
That might give some clues. If not it might be worth looking in the syslog after (it has suspend information far more detailed than the log above).
Can you suspend correctly (power off the monitor) from the suspend option in the menu ? Is it only when you close the lid (and not from the menu) that the monitor will not power down ?
What version of Ubuntu are you using ?
Please post the output of (this will tell us)