hmm dang! A few questions:
1) What OS version is your machine running?
2) Just to make sure, can you see "v1.2.2" on the Pixel Fireplace title screen at the bottom left?
Okay, that's what I'm on too. Would you mind trying out a debug version? It'll show the variables used for the displays and game-window xy positions, and sizes. You can email me at tedmartens [at] gmail [dot] com
Also, before going fullscreen pixel fireplace checks the top-left corner of it's window to determine which display to go to, so if the window isn't all the way within a secondary display it will go to the main display or whichever display that corner is within. It defaults to the main display if the top-left corner is outside of all display borders as well. Make sure you've dragged the window all the way to your secondary screen before trying fullscreen. (I might change it so that it checks the center of the window instead of the top-left corner)