A coach told me the same thing years back and I fought it for weeks because my kick felt fast. Turns out I was kicking from my knees and just churning water instead of getting any push. Once I switched to kicking from the hip with straighter legs my whole stroke smoothed out and my times dropped. Read somewhere that most age group swimmers lose more time to a bad kick than to bad arms, which surprised me at first. It sounds backwards but fixing the kick fixes everything downstream.