The team here at Signagelive have had reports that if the monitor is switched off when the Chromebox is switched on, sometimes the Signagelive output does not fill the entire screen. This is a known Chrome OS issue reported elsewhere. It has been raised with Google, however, the current status is that the issue is under review and we are awaiting a solution. The current resolution is to ensure that the display is powered on before the Chrome device powers on.
More information on the above topics can be found in the following forum articles: