I see that the window position was being constrained by an interior wall. I made that a half-wall (which is OK) and it no longer had a problem moving horizontally. But it seems that there has to be framing in the gable end for the window to be attached to. That is, I cannot move the window higher in the gable end since there is no framing member to attach it to there. Is that what is going on? I suppose there is a way to add specific wall framing, though I do not know how. Any advice?