Yeah I get them. Usually it's at end of the day when you don't feel tired but your legs truly are. Fatigue makes landings difficult. (You're resisting a lot of G's; try leg pressing 500 after a long run...) But other times there's just no reason. Maybe that morning you accidentally spread your toast with Vegemite instead of Marmite. Sometime's it's magic, sometime's it's tragic. Just blow it off and come back another day.