have you uploaded it to the falcon yet? ... might be in xlights, but if you haven't uploaded it to the controller, then it'll only be lighting up the default amount of pixels (or whatever that port might have previously been set to).
My bad, I didn't even think it could be a faulty string - but seems the second string (pixels 101-199) is faulty, it blows a fuse on the falcon when directly connected, even with no outputs. Swapped this out and all good.