Toning the abs/core

Hi fb community!

Recently, I've been having issues in trying to tone my stomach. I don't have much body fat relatively (thanks fitness blender!), I just can't seem to tone those abs (despite my legs and my arms showing definition and tone). I'm trying not to let it bother me - but it does, unfortunately. The two schools of thought in ab toning seem to be to do hiit/cardio and core workouts, although Kelli says often that full body workouts are better for your core than isolated ab workouts. Does anyone have any advice? Thanks!