The problem I see with many new and some more advanced Taekwondo students is improper technique when throwing a roundhouse kick. When performing the kick they will exert most of their power strictly from the lever action of the knee extending to the toes. At the same time their base foot is usually stationary and flat. If you think about it, there is only so much power that can be projected from the simple kicking action of the knee through to the toes.
So where exactly does the power of the roundhouse kick originate from? The real force comes from the swinging motion of the hips, while propelling the knee through your intended target. The extension of the bottom half of the leg (below the knee) is thus just a result of properly rotating and swinging the hips through the target, not the sole source of power and force. In the same way that kicking a ball from a stationary position would generate less power than walking into the kick, the same is true with the swinging of the hips and body to provide more power to the roundhouse kick.
When swinging the hips through the target, the stationary foot will now become mobile by the rotation on the ball of that foot. By the end of the kick, the stationary (support) foot should have it's heel facing the target (the foot rotating a full 180 degrees on the ball). The combination of the hip motion and rotation of the stationary foot will combine to bring a much more powerful, as well as accurate and controlled kicking technique.
Proper technique does not just occur upon the initial instruction. The student must be constantly reminded until it becomes second nature. At that point the student will be using his roundhouse kick at its fullest potential.