Following up on my previous post, the first reason that can cause flautists to cover too much of the embouchure hole is compensating for a lack of air. Instead of focussing on a good consistent airstream that is pliable and appropriate for what is being played, the flautist rolls the flute inwards, overworking the lips, attempting to produce a better sound. The higher up the flute they play the more the flute gets rolled in and the tighter the lips become. This produces a thin tone lacking projection as it reduces the harmonics present in the sound. Intonation becomes very unstable and has a tendency to become very sharp as you ascend through the registers. Additionally an over covered embouchure hole limits the dynamics that are possible; especially at the forte end of the range. So the thing I often tell my students is more air, less face.