[David R]

There are people around who can shoot the full weight long bows, and the best of them can achieve the Medieval rate of 6 arrows in the air at the same time......but it takes years of training, and a great toll on the archers.
Contemperary writers made much of the fact that soldier lost condition when in the field. Agincourt was fought because Henry knew he had no more time, another day or so and his men would not be up to par.
Perhaps the most vital factor was the availability of good enough wood, English factors scoured Europe for top grade Yew and to eke out supplies bowyers were ordered to make 3 bows of Ash or Elm for every one they made of Yew. To buy a top grade stave of Yew today will run you hundreds of pounds sterling, last quote I heard was about £400 for a best stave.
Finaly a change over in iron production that made for cheaper steel and it's more widespread use in armour, even in the munitions grade.
All stuff covered in a lecture I gave at Leeds Royal Armouries some years ago titled "Breaking Bows". XD