With maple (if only all wood could be that) my Fiskars X27 is usually all I need. I think that does better than my 8 lb. axe.Swinging a sledge hammer=workout
Try a Fiskers to prolong your swinging time
I think you hit the wedge (nail) on the head - it was in fact at an angle. So my technique came into play. I think this time the round (with knots) was on the ground since it was frozen. As for the height, not very - maybe 16 - 18".The temperature won't make much of a difference (19 F or 40 F...) for steel.
I suspect you accidentally hit it not straight on, but at an angle. ...