Or just sucking at it, like me. I have this horrible habit of mindlessly slashing in every direction like I'm playing TP, and for some reason I'm bad at stabbing in hero mode. I swear stabbing was easier in normal mode...
I also noticed stabbing is easier...NOT! Through several playthroughs I realized that I was subconsciously being more cautious with my movements in HM because no heart respawns and rupees are hard as hell to acquire. So maybe you're just being subconsciously cautious with HM as well?