I have had this issue several times, and resolved it by restarting, reclicking the right options etc. Now it's not working.
My Dell XPS M1530 runs Windows 8.1 with Start8. Occasionally, I will use an external monitor. Now the start menue is located exclusively on the external monitor again, despite always starting and shutting down windows with this monitor off. When I started my laptop now, there was no start menue at all on the screen. If I tell Start8 to display a start menue on all screens active, the menue itself will pop up on the external screen anyway, even when it's off. Windows and my nVidia graphic card correctly identify my laptop screen as #1, the external Dell monitor as #2.
So how do I make Start8 function when my external monitor is off, and how do I keep my start menue on the right screen at all times? This just changed without me doing any changes, there wasn't even any kind of software update (Start8, Windows, Firefox even...).